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Network Working Group                                         J. KlensinRequest for Comments: 5321                                  October 2008Obsoletes:2821Updates:1123Category: Standards TrackSimple Mail Transfer ProtocolStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This document is a specification of the basic protocol for Internet   electronic mail transport.  It consolidates, updates, and clarifies   several previous documents, making all or parts of most of them   obsolete.  It covers the SMTP extension mechanisms and best practices   for the contemporary Internet, but does not provide details about   particular extensions.  Although SMTP was designed as a mail   transport and delivery protocol, this specification also contains   information that is important to its use as a "mail submission"   protocol for "split-UA" (User Agent) mail reading systems and mobile   environments.Klensin                     Standards Track                     [Page 1]

RFC 5321                          SMTP                      October 2008Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .51.1.  Transport of Electronic Mail . . . . . . . . . . . . . . .51.2.  History and Context for This Document  . . . . . . . . . .51.3.  Document Conventions . . . . . . . . . . . . . . . . . . .62.  The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . .72.1.  Basic Structure  . . . . . . . . . . . . . . . . . . . . .72.2.  The Extension Model  . . . . . . . . . . . . . . . . . . .92.2.1.  Background . . . . . . . . . . . . . . . . . . . . . .92.2.2.  Definition and Registration of Extensions  . . . . . .102.2.3.  Special Issues with Extensions . . . . . . . . . . . .112.3.  SMTP Terminology . . . . . . . . . . . . . . . . . . . . .112.3.1.  Mail Objects . . . . . . . . . . . . . . . . . . . . .112.3.2.  Senders and Receivers  . . . . . . . . . . . . . . . .122.3.3.  Mail Agents and Message Stores . . . . . . . . . . . .122.3.4.  Host . . . . . . . . . . . . . . . . . . . . . . . . .132.3.5.  Domain Names . . . . . . . . . . . . . . . . . . . . .132.3.6.  Buffer and State Table . . . . . . . . . . . . . . . .142.3.7.  Commands and Replies . . . . . . . . . . . . . . . . .142.3.8.  Lines  . . . . . . . . . . . . . . . . . . . . . . . .142.3.9.  Message Content and Mail Data  . . . . . . . . . . . .152.3.10. Originator, Delivery, Relay, and Gateway Systems . . .152.3.11. Mailbox and Address  . . . . . . . . . . . . . . . . .152.4.  General Syntax Principles and Transaction Model  . . . . .163.  The SMTP Procedures: An Overview . . . . . . . . . . . . . . .173.1.  Session Initiation . . . . . . . . . . . . . . . . . . . .183.2.  Client Initiation  . . . . . . . . . . . . . . . . . . . .183.3.  Mail Transactions  . . . . . . . . . . . . . . . . . . . .193.4.  Forwarding for Address Correction or Updating  . . . . . .213.5.  Commands for Debugging Addresses . . . . . . . . . . . . .223.5.1.  Overview . . . . . . . . . . . . . . . . . . . . . . .223.5.2.  VRFY Normal Response . . . . . . . . . . . . . . . . .243.5.3.  Meaning of VRFY or EXPN Success Response . . . . . . .253.5.4.  Semantics and Applications of EXPN . . . . . . . . . .263.6.  Relaying and Mail Routing  . . . . . . . . . . . . . . . .263.6.1.  Source Routes and Relaying . . . . . . . . . . . . . .263.6.2.  Mail eXchange Records and Relaying . . . . . . . . . .263.6.3.  Message Submission Servers as Relays . . . . . . . . .273.7.  Mail Gatewaying  . . . . . . . . . . . . . . . . . . . . .283.7.1.  Header Fields in Gatewaying  . . . . . . . . . . . . .283.7.2.  Received Lines in Gatewaying . . . . . . . . . . . . .293.7.3.  Addresses in Gatewaying  . . . . . . . . . . . . . . .293.7.4.  Other Header Fields in Gatewaying  . . . . . . . . . .293.7.5.  Envelopes in Gatewaying  . . . . . . . . . . . . . . .303.8.  Terminating Sessions and Connections . . . . . . . . . . .303.9.  Mailing Lists and Aliases  . . . . . . . . . . . . . . . .313.9.1.  Alias  . . . . . . . . . . . . . . . . . . . . . . . .31Klensin                     Standards Track                     [Page 2]

RFC 5321                          SMTP                      October 20083.9.2.  List . . . . . . . . . . . . . . . . . . . . . . . . .314.  The SMTP Specifications  . . . . . . . . . . . . . . . . . . .324.1.  SMTP Commands  . . . . . . . . . . . . . . . . . . . . . .324.1.1.  Command Semantics and Syntax . . . . . . . . . . . . .324.1.2.  Command Argument Syntax  . . . . . . . . . . . . . . .414.1.3.  Address Literals . . . . . . . . . . . . . . . . . . .434.1.4.  Order of Commands  . . . . . . . . . . . . . . . . . .444.1.5.  Private-Use Commands . . . . . . . . . . . . . . . . .464.2.  SMTP Replies . . . . . . . . . . . . . . . . . . . . . . .464.2.1.  Reply Code Severities and Theory . . . . . . . . . . .484.2.2.  Reply Codes by Function Groups . . . . . . . . . . . .504.2.3.  Reply Codes in Numeric Order . . . . . . . . . . . . .524.2.4.  Reply Code 502 . . . . . . . . . . . . . . . . . . . .53       4.2.5.  Reply Codes after DATA and the Subsequent               <CRLF>.<CRLF>  . . . . . . . . . . . . . . . . . . . .534.3.  Sequencing of Commands and Replies . . . . . . . . . . . .544.3.1.  Sequencing Overview  . . . . . . . . . . . . . . . . .544.3.2.  Command-Reply Sequences  . . . . . . . . . . . . . . .554.4.  Trace Information  . . . . . . . . . . . . . . . . . . . .574.5.  Additional Implementation Issues . . . . . . . . . . . . .614.5.1.  Minimum Implementation . . . . . . . . . . . . . . . .614.5.2.  Transparency . . . . . . . . . . . . . . . . . . . . .624.5.3.  Sizes and Timeouts . . . . . . . . . . . . . . . . . .624.5.3.1.  Size Limits and Minimums . . . . . . . . . . . . .624.5.3.1.1.  Local-part . . . . . . . . . . . . . . . . . .634.5.3.1.2.  Domain . . . . . . . . . . . . . . . . . . . .634.5.3.1.3.  Path . . . . . . . . . . . . . . . . . . . . .634.5.3.1.4.  Command Line . . . . . . . . . . . . . . . . .634.5.3.1.5.  Reply Line . . . . . . . . . . . . . . . . . .634.5.3.1.6.  Text Line  . . . . . . . . . . . . . . . . . .634.5.3.1.7.  Message Content  . . . . . . . . . . . . . . .634.5.3.1.8.  Recipients Buffer  . . . . . . . . . . . . . .644.5.3.1.9.  Treatment When Limits Exceeded . . . . . . . .644.5.3.1.10. Too Many Recipients Code . . . . . . . . . . .644.5.3.2.  Timeouts . . . . . . . . . . . . . . . . . . . . .654.5.3.2.1.  Initial 220 Message: 5 Minutes . . . . . . . .654.5.3.2.2.  MAIL Command: 5 Minutes  . . . . . . . . . . .654.5.3.2.3.  RCPT Command: 5 Minutes  . . . . . . . . . . .654.5.3.2.4.  DATA Initiation: 2 Minutes . . . . . . . . . .664.5.3.2.5.  Data Block: 3 Minutes  . . . . . . . . . . . .66           4.5.3.2.6.  DATA Termination: 10 Minutes.  . . . . . . . .664.5.3.2.7.  Server Timeout: 5 Minutes. . . . . . . . . . .664.5.4.  Retry Strategies . . . . . . . . . . . . . . . . . . .664.5.5.  Messages with a Null Reverse-Path  . . . . . . . . . .685.  Address Resolution and Mail Handling . . . . . . . . . . . . .695.1.  Locating the Target Host . . . . . . . . . . . . . . . . .695.2.  IPv6 and MX Records  . . . . . . . . . . . . . . . . . . .716.  Problem Detection and Handling . . . . . . . . . . . . . . . .71Klensin                     Standards Track                     [Page 3]

RFC 5321                          SMTP                      October 20086.1.  Reliable Delivery and Replies by Email . . . . . . . . . .716.2.  Unwanted, Unsolicited, and "Attack" Messages . . . . . . .726.3.  Loop Detection . . . . . . . . . . . . . . . . . . . . . .736.4.  Compensating for Irregularities  . . . . . . . . . . . . .737.  Security Considerations  . . . . . . . . . . . . . . . . . . .757.1.  Mail Security and Spoofing . . . . . . . . . . . . . . . .757.2.  "Blind" Copies . . . . . . . . . . . . . . . . . . . . . .767.3.  VRFY, EXPN, and Security . . . . . . . . . . . . . . . . .767.4.  Mail Rerouting Based on the 251 and 551 Response Codes . .777.5.  Information Disclosure in Announcements  . . . . . . . . .777.6.  Information Disclosure in Trace Fields . . . . . . . . . .787.7.  Information Disclosure in Message Forwarding . . . . . . .787.8.  Resistance to Attacks  . . . . . . . . . . . . . . . . . .787.9.  Scope of Operation of SMTP Servers . . . . . . . . . . . .788.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .799.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .8010. References . . . . . . . . . . . . . . . . . . . . . . . . . .8110.1. Normative References . . . . . . . . . . . . . . . . . . .8110.2. Informative References . . . . . . . . . . . . . . . . . .82Appendix A.  TCP Transport Service . . . . . . . . . . . . . . . .85Appendix B.  Generating SMTP Commands fromRFC 822 Header                Fields  . . . . . . . . . . . . . . . . . . . . . . .85Appendix C.  Source Routes . . . . . . . . . . . . . . . . . . . .86Appendix D.  Scenarios . . . . . . . . . . . . . . . . . . . . . .87D.1.  A Typical SMTP Transaction Scenario  . . . . . . . . . . .88D.2.  Aborted SMTP Transaction Scenario  . . . . . . . . . . . .89D.3.  Relayed Mail Scenario  . . . . . . . . . . . . . . . . . .90D.4.  Verifying and Sending Scenario . . . . . . . . . . . . . .92Appendix E.  Other Gateway Issues  . . . . . . . . . . . . . . . .92Appendix F.  Deprecated Features ofRFC 821  . . . . . . . . . . .93F.1.  TURN . . . . . . . . . . . . . . . . . . . . . . . . . . .93F.2.  Source Routing . . . . . . . . . . . . . . . . . . . . . .93F.3.  HELO . . . . . . . . . . . . . . . . . . . . . . . . . . .93F.4.  #-literals . . . . . . . . . . . . . . . . . . . . . . . .94F.5.  Dates and Years  . . . . . . . . . . . . . . . . . . . . .94F.6.  Sending versus Mailing . . . . . . . . . . . . . . . . . .94Klensin                     Standards Track                     [Page 4]

RFC 5321                          SMTP                      October 20081.  Introduction1.1.  Transport of Electronic Mail   The objective of the Simple Mail Transfer Protocol (SMTP) is to   transfer mail reliably and efficiently.   SMTP is independent of the particular transmission subsystem and   requires only a reliable ordered data stream channel.  While this   document specifically discusses transport over TCP, other transports   are possible.  Appendices toRFC 821 [1] describe some of them.   An important feature of SMTP is its capability to transport mail   across multiple networks, usually referred to as "SMTP mail relaying"   (seeSection 3.6).  A network consists of the mutually-TCP-accessible   hosts on the public Internet, the mutually-TCP-accessible hosts on a   firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN   environment utilizing a non-TCP transport-level protocol.  Using   SMTP, a process can transfer mail to another process on the same   network or to some other network via a relay or gateway process   accessible to both networks.   In this way, a mail message may pass through a number of intermediate   relay or gateway hosts on its path from sender to ultimate recipient.   The Mail eXchanger mechanisms of the domain name system (RFC 1035   [2],RFC 974 [12], andSection 5 of this document) are used to   identify the appropriate next-hop destination for a message being   transported.1.2.  History and Context for This Document   This document is a specification of the basic protocol for the   Internet electronic mail transport.  It consolidates, updates and   clarifies, but does not add new or change existing functionality of   the following:   o  the original SMTP (Simple Mail Transfer Protocol) specification ofRFC 821 [1],   o  domain name system requirements and implications for mail      transport fromRFC 1035 [2] andRFC 974 [12],   o  the clarifications and applicability statements inRFC 1123 [3],      and   o  material drawn from the SMTP Extension mechanisms inRFC 1869      [13].Klensin                     Standards Track                     [Page 5]

RFC 5321                          SMTP                      October 2008   o  Editorial and clarification changes toRFC 2821 [14] to bring that      specification to Draft Standard.   It obsoletesRFC 821,RFC 974,RFC 1869, andRFC 2821 and updatesRFC1123 (replacing the mail transport materials ofRFC 1123).  However,RFC 821 specifies some features that were not in significant use in   the Internet by the mid-1990s and (in appendices) some additional   transport models.  Those sections are omitted here in the interest of   clarity and brevity; readers needing them should refer toRFC 821.   It also includes some additional material fromRFC 1123 that required   amplification.  This material has been identified in multiple ways,   mostly by tracking flaming on various lists and newsgroups and   problems of unusual readings or interpretations that have appeared as   the SMTP extensions have been deployed.  Where this specification   moves beyond consolidation and actually differs from earlier   documents, it supersedes them technically as well as textually.   Although SMTP was designed as a mail transport and delivery protocol,   this specification also contains information that is important to its   use as a "mail submission" protocol, as recommended for Post Office   Protocol (POP) (RFC 937 [15],RFC 1939 [16]) and IMAP (RFC 3501   [17]).  In general, the separate mail submission protocol specified   inRFC 4409 [18] is now preferred to direct use of SMTP; more   discussion of that subject appears in that document.Section 2.3 provides definitions of terms specific to this document.   Except when the historical terminology is necessary for clarity, this   document uses the current 'client' and 'server' terminology to   identify the sending and receiving SMTP processes, respectively.   A companion document,RFC 5322 [4], discusses message header sections   and bodies and specifies formats and structures for them.1.3.  Document Conventions   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [5].  As each   of these terms was intentionally and carefully chosen to improve the   interoperability of email, each use of these terms is to be treated   as a conformance requirement.   Because this document has a long history and to avoid the risk of   various errors and of confusing readers and documents that point to   this one, most examples and the domain names they contain are   preserved fromRFC 2821.  Readers are cautioned that these areKlensin                     Standards Track                     [Page 6]

RFC 5321                          SMTP                      October 2008   illustrative examples that should not actually be used in either code   or configuration files.2.  The SMTP Model2.1.  Basic Structure   The SMTP design can be pictured as:                  +----------+                +----------+      +------+    |          |                |          |      | User |<-->|          |      SMTP      |          |      +------+    |  Client- |Commands/Replies| Server-  |      +------+    |   SMTP   |<-------------->|    SMTP  |    +------+      | File |<-->|          |    and Mail    |          |<-->| File |      |System|    |          |                |          |    |System|      +------+    +----------+                +----------+    +------+                   SMTP client                SMTP server   When an SMTP client has a message to transmit, it establishes a two-   way transmission channel to an SMTP server.  The responsibility of an   SMTP client is to transfer mail messages to one or more SMTP servers,   or report its failure to do so.   The means by which a mail message is presented to an SMTP client, and   how that client determines the identifier(s) ("names") of the   domain(s) to which mail messages are to be transferred, is a local   matter, and is not addressed by this document.  In some cases, the   designated domain(s), or those determined by an SMTP client, will   identify the final destination(s) of the mail message.  In other   cases, common with SMTP clients associated with implementations of   the POP (RFC 937 [15],RFC 1939 [16]) or IMAP (RFC 3501 [17])   protocols, or when the SMTP client is inside an isolated transport   service environment, the domain determined will identify an   intermediate destination through which all mail messages are to be   relayed.  SMTP clients that transfer all traffic regardless of the   target domains associated with the individual messages, or that do   not maintain queues for retrying message transmissions that initially   cannot be completed, may otherwise conform to this specification but   are not considered fully-capable.  Fully-capable SMTP   implementations, including the relays used by these less capable   ones, and their destinations, are expected to support all of the   queuing, retrying, and alternate address functions discussed in this   specification.  In many situations and configurations, the less-   capable clients discussed above SHOULD be using the message   submission protocol (RFC 4409 [18]) rather than SMTP.Klensin                     Standards Track                     [Page 7]

RFC 5321                          SMTP                      October 2008   The means by which an SMTP client, once it has determined a target   domain, determines the identity of an SMTP server to which a copy of   a message is to be transferred, and then performs that transfer, is   covered by this document.  To effect a mail transfer to an SMTP   server, an SMTP client establishes a two-way transmission channel to   that SMTP server.  An SMTP client determines the address of an   appropriate host running an SMTP server by resolving a destination   domain name to either an intermediate Mail eXchanger host or a final   target host.   An SMTP server may be either the ultimate destination or an   intermediate "relay" (that is, it may assume the role of an SMTP   client after receiving the message) or "gateway" (that is, it may   transport the message further using some protocol other than SMTP).   SMTP commands are generated by the SMTP client and sent to the SMTP   server.  SMTP replies are sent from the SMTP server to the SMTP   client in response to the commands.   In other words, message transfer can occur in a single connection   between the original SMTP-sender and the final SMTP-recipient, or can   occur in a series of hops through intermediary systems.  In either   case, once the server has issued a success response at the end of the   mail data, a formal handoff of responsibility for the message occurs:   the protocol requires that a server MUST accept responsibility for   either delivering the message or properly reporting the failure to do   so (see Sections6.1,6.2, and7.8, below).   Once the transmission channel is established and initial handshaking   is completed, the SMTP client normally initiates a mail transaction.   Such a transaction consists of a series of commands to specify the   originator and destination of the mail and transmission of the   message content (including any lines in the header section or other   structure) itself.  When the same message is sent to multiple   recipients, this protocol encourages the transmission of only one   copy of the data for all recipients at the same destination (or   intermediate relay) host.   The server responds to each command with a reply; replies may   indicate that the command was accepted, that additional commands are   expected, or that a temporary or permanent error condition exists.   Commands specifying the sender or recipients may include server-   permitted SMTP service extension requests, as discussed inSection 2.2.  The dialog is purposely lock-step, one-at-a-time,   although this can be modified by mutually agreed upon extension   requests such as command pipelining (RFC 2920 [19]).   Once a given mail message has been transmitted, the client may either   request that the connection be shut down or may initiate other mailKlensin                     Standards Track                     [Page 8]

RFC 5321                          SMTP                      October 2008   transactions.  In addition, an SMTP client may use a connection to an   SMTP server for ancillary services such as verification of email   addresses or retrieval of mailing list subscriber addresses.   As suggested above, this protocol provides mechanisms for the   transmission of mail.  Historically, this transmission normally   occurred directly from the sending user's host to the receiving   user's host when the two hosts are connected to the same transport   service.  When they are not connected to the same transport service,   transmission occurs via one or more relay SMTP servers.  A very   common case in the Internet today involves submission of the original   message to an intermediate, "message submission" server, which is   similar to a relay but has some additional properties; such servers   are discussed inSection 2.3.10 and at some length inRFC 4409 [18].   An intermediate host that acts as either an SMTP relay or as a   gateway into some other transmission environment is usually selected   through the use of the domain name service (DNS) Mail eXchanger   mechanism.   Usually, intermediate hosts are determined via the DNS MX record, not   by explicit "source" routing (seeSection 5 andAppendix C andAppendix F.2).2.2.  The Extension Model2.2.1.  Background   In an effort that started in 1990, approximately a decade afterRFC821 was completed, the protocol was modified with a "service   extensions" model that permits the client and server to agree to   utilize shared functionality beyond the original SMTP requirements.   The SMTP extension mechanism defines a means whereby an extended SMTP   client and server may recognize each other, and the server can inform   the client as to the service extensions that it supports.   Contemporary SMTP implementations MUST support the basic extension   mechanisms.  For instance, servers MUST support the EHLO command even   if they do not implement any specific extensions and clients SHOULD   preferentially utilize EHLO rather than HELO.  (However, for   compatibility with older conforming implementations, SMTP clients and   servers MUST support the original HELO mechanisms as a fallback.)   Unless the different characteristics of HELO must be identified for   interoperability purposes, this document discusses only EHLO.   SMTP is widely deployed and high-quality implementations have proven   to be very robust.  However, the Internet community now considers   some services to be important that were not anticipated when the   protocol was first designed.  If support for those services is to beKlensin                     Standards Track                     [Page 9]

RFC 5321                          SMTP                      October 2008   added, it must be done in a way that permits older implementations to   continue working acceptably.  The extension framework consists of:   o  The SMTP command EHLO, superseding the earlier HELO,   o  a registry of SMTP service extensions,   o  additional parameters to the SMTP MAIL and RCPT commands, and   o  optional replacements for commands defined in this protocol, such      as for DATA in non-ASCII transmissions (RFC 3030 [20]).   SMTP's strength comes primarily from its simplicity.  Experience with   many protocols has shown that protocols with few options tend towards   ubiquity, whereas protocols with many options tend towards obscurity.   Each and every extension, regardless of its benefits, must be   carefully scrutinized with respect to its implementation, deployment,   and interoperability costs.  In many cases, the cost of extending the   SMTP service will likely outweigh the benefit.2.2.2.  Definition and Registration of Extensions   The IANA maintains a registry of SMTP service extensions.  A   corresponding EHLO keyword value is associated with each extension.   Each service extension registered with the IANA must be defined in a   formal Standards-Track or IESG-approved Experimental protocol   document.  The definition must include:   o  the textual name of the SMTP service extension;   o  the EHLO keyword value associated with the extension;   o  the syntax and possible values of parameters associated with the      EHLO keyword value;   o  any additional SMTP verbs associated with the extension      (additional verbs will usually be, but are not required to be, the      same as the EHLO keyword value);   o  any new parameters the extension associates with the MAIL or RCPT      verbs;   o  a description of how support for the extension affects the      behavior of a server and client SMTP; andKlensin                     Standards Track                    [Page 10]

RFC 5321                          SMTP                      October 2008   o  the increment by which the extension is increasing the maximum      length of the commands MAIL and/or RCPT, over that specified in      this Standard.   In addition, any EHLO keyword value starting with an upper or lower   case "X" refers to a local SMTP service extension used exclusively   through bilateral agreement.  Keywords beginning with "X" MUST NOT be   used in a registered service extension.  Conversely, keyword values   presented in the EHLO response that do not begin with "X" MUST   correspond to a Standard, Standards-Track, or IESG-approved   Experimental SMTP service extension registered with IANA.  A   conforming server MUST NOT offer non-"X"-prefixed keyword values that   are not described in a registered extension.   Additional verbs and parameter names are bound by the same rules as   EHLO keywords; specifically, verbs beginning with "X" are local   extensions that may not be registered or standardized.  Conversely,   verbs not beginning with "X" must always be registered.2.2.3.  Special Issues with Extensions   Extensions that change fairly basic properties of SMTP operation are   permitted.  The text in other sections of this document must be   understood in that context.  In particular, extensions can change the   minimum limits specified inSection 4.5.3, can change the ASCII   character set requirement as mentioned above, or can introduce some   optional modes of message handling.   In particular, if an extension implies that the delivery path   normally supports special features of that extension, and an   intermediate SMTP system finds a next hop that does not support the   required extension, it MAY choose, based on the specific extension   and circumstances, to requeue the message and try later and/or try an   alternate MX host.  If this strategy is employed, the timeout to fall   back to an unextended format (if one is available) SHOULD be less   than the normal timeout for bouncing as undeliverable (e.g., if   normal timeout is three days, the requeue timeout before attempting   to transmit the mail without the extension might be one day).2.3.  SMTP Terminology2.3.1.  Mail Objects   SMTP transports a mail object.  A mail object contains an envelope   and content.   The SMTP envelope is sent as a series of SMTP protocol units   (described inSection 3).  It consists of an originator address (toKlensin                     Standards Track                    [Page 11]

RFC 5321                          SMTP                      October 2008   which error reports should be directed), one or more recipient   addresses, and optional protocol extension material.  Historically,   variations on the reverse-path (originator) address specification   command (MAIL) could be used to specify alternate delivery modes,   such as immediate display; those variations have now been deprecated   (seeAppendix F andAppendix F.6).   The SMTP content is sent in the SMTP DATA protocol unit and has two   parts: the header section and the body.  If the content conforms to   other contemporary standards, the header section consists of a   collection of header fields, each consisting of a header name, a   colon, and data, structured as in the message format specification   (RFC 5322 [4]); the body, if structured, is defined according to MIME   (RFC 2045 [21]).  The content is textual in nature, expressed using   the US-ASCII repertoire [6].  Although SMTP extensions (such as   "8BITMIME",RFC 1652 [22]) may relax this restriction for the content   body, the content header fields are always encoded using the US-ASCII   repertoire.  Two MIME extensions (RFC 2047 [23] andRFC 2231 [24])   define an algorithm for representing header values outside the US-   ASCII repertoire, while still encoding them using the US-ASCII   repertoire.2.3.2.  Senders and Receivers   InRFC 821, the two hosts participating in an SMTP transaction were   described as the "SMTP-sender" and "SMTP-receiver".  This document   has been changed to reflect current industry terminology and hence   refers to them as the "SMTP client" (or sometimes just "the client")   and "SMTP server" (or just "the server"), respectively.  Since a   given host may act both as server and client in a relay situation,   "receiver" and "sender" terminology is still used where needed for   clarity.2.3.3.  Mail Agents and Message Stores   Additional mail system terminology became common afterRFC 821 was   published and, where convenient, is used in this specification.  In   particular, SMTP servers and clients provide a mail transport service   and therefore act as "Mail Transfer Agents" (MTAs).  "Mail User   Agents" (MUAs or UAs) are normally thought of as the sources and   targets of mail.  At the source, an MUA might collect mail to be   transmitted from a user and hand it off to an MTA; the final   ("delivery") MTA would be thought of as handing the mail off to an   MUA (or at least transferring responsibility to it, e.g., by   depositing the message in a "message store").  However, while these   terms are used with at least the appearance of great precision in   other environments, the implied boundaries between MUAs and MTAs   often do not accurately match common, and conforming, practices withKlensin                     Standards Track                    [Page 12]

RFC 5321                          SMTP                      October 2008   Internet mail.  Hence, the reader should be cautious about inferring   the strong relationships and responsibilities that might be implied   if these terms were used elsewhere.2.3.4.  Host   For the purposes of this specification, a host is a computer system   attached to the Internet (or, in some cases, to a private TCP/IP   network) and supporting the SMTP protocol.  Hosts are known by names   (see the next section); they SHOULD NOT be identified by numerical   addresses, i.e., by address literals as described inSection 4.1.2.2.3.5.  Domain Names   A domain name (or often just a "domain") consists of one or more   components, separated by dots if more than one appears.  In the case   of a top-level domain used by itself in an email address, a single   string is used without any dots.  This makes the requirement,   described in more detail below, that only fully-qualified domain   names appear in SMTP transactions on the public Internet,   particularly important where top-level domains are involved.  These   components ("labels" in DNS terminology,RFC 1035 [2]) are restricted   for SMTP purposes to consist of a sequence of letters, digits, and   hyphens drawn from the ASCII character set [6].  Domain names are   used as names of hosts and of other entities in the domain name   hierarchy.  For example, a domain may refer to an alias (label of a   CNAME RR) or the label of Mail eXchanger records to be used to   deliver mail instead of representing a host name.  SeeRFC 1035 [2]   andSection 5 of this specification.   The domain name, as described in this document and inRFC 1035 [2],   is the entire, fully-qualified name (often referred to as an "FQDN").   A domain name that is not in FQDN form is no more than a local alias.   Local aliases MUST NOT appear in any SMTP transaction.   Only resolvable, fully-qualified domain names (FQDNs) are permitted   when domain names are used in SMTP.  In other words, names that can   be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed   inSection 5) are permitted, as are CNAME RRs whose targets can be   resolved, in turn, to MX or address RRs.  Local nicknames or   unqualified names MUST NOT be used.  There are two exceptions to the   rule requiring FQDNs:   o  The domain name given in the EHLO command MUST be either a primary      host name (a domain name that resolves to an address RR) or, if      the host has no name, an address literal, as described inSection 4.1.3 and discussed further in the EHLO discussion ofSection 4.1.4.Klensin                     Standards Track                    [Page 13]

RFC 5321                          SMTP                      October 2008   o  The reserved mailbox name "postmaster" may be used in a RCPT      command without domain qualification (seeSection 4.1.1.3) and      MUST be accepted if so used.2.3.6.  Buffer and State Table   SMTP sessions are stateful, with both parties carefully maintaining a   common view of the current state.  In this document, we model this   state by a virtual "buffer" and a "state table" on the server that   may be used by the client to, for example, "clear the buffer" or   "reset the state table", causing the information in the buffer to be   discarded and the state to be returned to some previous state.2.3.7.  Commands and Replies   SMTP commands and, unless altered by a service extension, message   data, are transmitted from the sender to the receiver via the   transmission channel in "lines".   An SMTP reply is an acknowledgment (positive or negative) sent in   "lines" from receiver to sender via the transmission channel in   response to a command.  The general form of a reply is a numeric   completion code (indicating failure or success) usually followed by a   text string.  The codes are for use by programs and the text is   usually intended for human users.RFC 3463 [25], specifies further   structuring of the reply strings, including the use of supplemental   and more specific completion codes (see alsoRFC 5248 [26]).2.3.8.  Lines   Lines consist of zero or more data characters terminated by the   sequence ASCII character "CR" (hex value 0D) followed immediately by   ASCII character "LF" (hex value 0A).  This termination sequence is   denoted as <CRLF> in this document.  Conforming implementations MUST   NOT recognize or generate any other character or character sequence   as a line terminator.  Limits MAY be imposed on line lengths by   servers (seeSection 4).   In addition, the appearance of "bare" "CR" or "LF" characters in text   (i.e., either without the other) has a long history of causing   problems in mail implementations and applications that use the mail   system as a tool.  SMTP client implementations MUST NOT transmit   these characters except when they are intended as line terminators   and then MUST, as indicated above, transmit them only as a <CRLF>   sequence.Klensin                     Standards Track                    [Page 14]

RFC 5321                          SMTP                      October 20082.3.9.  Message Content and Mail Data   The terms "message content" and "mail data" are used interchangeably   in this document to describe the material transmitted after the DATA   command is accepted and before the end of data indication is   transmitted.  Message content includes the message header section and   the possibly structured message body.  The MIME specification (RFC2045 [21]) provides the standard mechanisms for structured message   bodies.2.3.10.  Originator, Delivery, Relay, and Gateway Systems   This specification makes a distinction among four types of SMTP   systems, based on the role those systems play in transmitting   electronic mail.  An "originating" system (sometimes called an SMTP   originator) introduces mail into the Internet or, more generally,   into a transport service environment.  A "delivery" SMTP system is   one that receives mail from a transport service environment and   passes it to a mail user agent or deposits it in a message store that   a mail user agent is expected to subsequently access.  A "relay" SMTP   system (usually referred to just as a "relay") receives mail from an   SMTP client and transmits it, without modification to the message   data other than adding trace information, to another SMTP server for   further relaying or for delivery.   A "gateway" SMTP system (usually referred to just as a "gateway")   receives mail from a client system in one transport environment and   transmits it to a server system in another transport environment.   Differences in protocols or message semantics between the transport   environments on either side of a gateway may require that the gateway   system perform transformations to the message that are not permitted   to SMTP relay systems.  For the purposes of this specification,   firewalls that rewrite addresses should be considered as gateways,   even if SMTP is used on both sides of them (seeRFC 2979 [27]).2.3.11.  Mailbox and Address   As used in this specification, an "address" is a character string   that identifies a user to whom mail will be sent or a location into   which mail will be deposited.  The term "mailbox" refers to that   depository.  The two terms are typically used interchangeably unless   the distinction between the location in which mail is placed (the   mailbox) and a reference to it (the address) is important.  An   address normally consists of user and domain specifications.  The   standard mailbox naming convention is defined to be   "local-part@domain"; contemporary usage permits a much broader set of   applications than simple "user names".  Consequently, and due to a   long history of problems when intermediate hosts have attempted toKlensin                     Standards Track                    [Page 15]

RFC 5321                          SMTP                      October 2008   optimize transport by modifying them, the local-part MUST be   interpreted and assigned semantics only by the host specified in the   domain part of the address.2.4.  General Syntax Principles and Transaction Model   SMTP commands and replies have a rigid syntax.  All commands begin   with a command verb.  All replies begin with a three digit numeric   code.  In some commands and replies, arguments are required following   the verb or reply code.  Some commands do not accept arguments (after   the verb), and some reply codes are followed, sometimes optionally,   by free form text.  In both cases, where text appears, it is   separated from the verb or reply code by a space character.  Complete   definitions of commands and replies appear inSection 4.   Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command   and extension name keywords) are not case sensitive, with the sole   exception in this specification of a mailbox local-part (SMTP   Extensions may explicitly specify case-sensitive elements).  That is,   a command verb, an argument value other than a mailbox local-part,   and free form text MAY be encoded in upper case, lower case, or any   mixture of upper and lower case with no impact on its meaning.  The   local-part of a mailbox MUST BE treated as case sensitive.   Therefore, SMTP implementations MUST take care to preserve the case   of mailbox local-parts.  In particular, for some hosts, the user   "smith" is different from the user "Smith".  However, exploiting the   case sensitivity of mailbox local-parts impedes interoperability and   is discouraged.  Mailbox domains follow normal DNS rules and are   hence not case sensitive.   A few SMTP servers, in violation of this specification (andRFC 821)   require that command verbs be encoded by clients in upper case.   Implementations MAY wish to employ this encoding to accommodate those   servers.   The argument clause consists of a variable-length character string   ending with the end of the line, i.e., with the character sequence   <CRLF>.  The receiver will take no action until this sequence is   received.   The syntax for each command is shown with the discussion of that   command.  Common elements and parameters are shown inSection 4.1.2.   Commands and replies are composed of characters from the ASCII   character set [6].  When the transport service provides an 8-bit byte   (octet) transmission channel, each 7-bit character is transmitted,   right justified, in an octet with the high-order bit cleared to zero.   More specifically, the unextended SMTP service provides 7-bitKlensin                     Standards Track                    [Page 16]

RFC 5321                          SMTP                      October 2008   transport only.  An originating SMTP client that has not successfully   negotiated an appropriate extension with a particular server (see the   next paragraph) MUST NOT transmit messages with information in the   high-order bit of octets.  If such messages are transmitted in   violation of this rule, receiving SMTP servers MAY clear the high-   order bit or reject the message as invalid.  In general, a relay SMTP   SHOULD assume that the message content it has received is valid and,   assuming that the envelope permits doing so, relay it without   inspecting that content.  Of course, if the content is mislabeled and   the data path cannot accept the actual content, this may result in   the ultimate delivery of a severely garbled message to the recipient.   Delivery SMTP systems MAY reject such messages, or return them as   undeliverable, rather than deliver them.  In the absence of a server-   offered extension explicitly permitting it, a sending SMTP system is   not permitted to send envelope commands in any character set other   than US-ASCII.  Receiving systems SHOULD reject such commands,   normally using "500 syntax error - invalid character" replies.   8-bit message content transmission MAY be requested of the server by   a client using extended SMTP facilities, notably the "8BITMIME"   extension,RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP   servers.  However, it MUST NOT be construed as authorization to   transmit unrestricted 8-bit material, nor does 8BITMIME authorize   transmission of any envelope material in other than ASCII. 8BITMIME   MUST NOT be requested by senders for material with the high bit on   that is not in MIME format with an appropriate content-transfer   encoding; servers MAY reject such messages.   The metalinguistic notation used in this document corresponds to the   "Augmented BNF" used in other Internet mail system documents.  The   reader who is not familiar with that syntax should consult the ABNF   specification inRFC 5234 [7].  Metalanguage terms used in running   text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.   The reader is cautioned that the grammar expressed in the   metalanguage is not comprehensive.  There are many instances in which   provisions in the text constrain or otherwise modify the syntax or   semantics implied by the grammar.3.  The SMTP Procedures: An Overview   This section contains descriptions of the procedures used in SMTP:   session initiation, mail transaction, forwarding mail, verifying   mailbox names and expanding mailing lists, and opening and closing   exchanges.  Comments on relaying, a note on mail domains, and a   discussion of changing roles are included at the end of this section.   Several complete scenarios are presented inAppendix D.Klensin                     Standards Track                    [Page 17]

RFC 5321                          SMTP                      October 20083.1.  Session Initiation   An SMTP session is initiated when a client opens a connection to a   server and the server responds with an opening message.   SMTP server implementations MAY include identification of their   software and version information in the connection greeting reply   after the 220 code, a practice that permits more efficient isolation   and repair of any problems.  Implementations MAY make provision for   SMTP servers to disable the software and version announcement where   it causes security concerns.  While some systems also identify their   contact point for mail problems, this is not a substitute for   maintaining the required "postmaster" address (seeSection 4).   The SMTP protocol allows a server to formally reject a mail session   while still allowing the initial connection as follows: a 554   response MAY be given in the initial connection opening message   instead of the 220.  A server taking this approach MUST still wait   for the client to send a QUIT (seeSection 4.1.1.10) before closing   the connection and SHOULD respond to any intervening commands with   "503 bad sequence of commands".  Since an attempt to make an SMTP   connection to such a system is probably in error, a server returning   a 554 response on connection opening SHOULD provide enough   information in the reply text to facilitate debugging of the sending   system.3.2.  Client Initiation   Once the server has sent the greeting (welcoming) message and the   client has received it, the client normally sends the EHLO command to   the server, indicating the client's identity.  In addition to opening   the session, use of EHLO indicates that the client is able to process   service extensions and requests that the server provide a list of the   extensions it supports.  Older SMTP systems that are unable to   support service extensions, and contemporary clients that do not   require service extensions in the mail session being initiated, MAY   use HELO instead of EHLO.  Servers MUST NOT return the extended EHLO-   style response to a HELO command.  For a particular connection   attempt, if the server returns a "command not recognized" response to   EHLO, the client SHOULD be able to fall back and send HELO.   In the EHLO command, the host sending the command identifies itself;   the command may be interpreted as saying "Hello, I am <domain>" (and,   in the case of EHLO, "and I support service extension requests").Klensin                     Standards Track                    [Page 18]

RFC 5321                          SMTP                      October 20083.3.  Mail Transactions   There are three steps to SMTP mail transactions.  The transaction   starts with a MAIL command that gives the sender identification.  (In   general, the MAIL command may be sent only when no mail transaction   is in progress; seeSection 4.1.4.)  A series of one or more RCPT   commands follows, giving the receiver information.  Then, a DATA   command initiates transfer of the mail data and is terminated by the   "end of mail" data indicator, which also confirms the transaction.   The first step in the procedure is the MAIL command.      MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>   This command tells the SMTP-receiver that a new mail transaction is   starting and to reset all its state tables and buffers, including any   recipients or mail data.  The <reverse-path> portion of the first or   only argument contains the source mailbox (between "<" and ">"   brackets), which can be used to report errors (seeSection 4.2 for a   discussion of error reporting).  If accepted, the SMTP server returns   a "250 OK" reply.  If the mailbox specification is not acceptable for   some reason, the server MUST return a reply indicating whether the   failure is permanent (i.e., will occur again if the client tries to   send the same address again) or temporary (i.e., the address might be   accepted if the client tries again later).  Despite the apparent   scope of this requirement, there are circumstances in which the   acceptability of the reverse-path may not be determined until one or   more forward-paths (in RCPT commands) can be examined.  In those   cases, the server MAY reasonably accept the reverse-path (with a 250   reply) and then report problems after the forward-paths are received   and examined.  Normally, failures produce 550 or 553 replies.   Historically, the <reverse-path> was permitted to contain more than   just a mailbox; however, contemporary systems SHOULD NOT use source   routing (seeAppendix C).   The optional <mail-parameters> are associated with negotiated SMTP   service extensions (seeSection 2.2).   The second step in the procedure is the RCPT command.  This step of   the procedure can be repeated any number of times.      RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>   The first or only argument to this command includes a forward-path   (normally a mailbox and domain, always surrounded by "<" and ">"   brackets) identifying one recipient.  If accepted, the SMTP server   returns a "250 OK" reply and stores the forward-path.  If theKlensin                     Standards Track                    [Page 19]

RFC 5321                          SMTP                      October 2008   recipient is known not to be a deliverable address, the SMTP server   returns a 550 reply, typically with a string such as "no such user -   " and the mailbox name (other circumstances and reply codes are   possible).   The <forward-path> can contain more than just a mailbox.   Historically, the <forward-path> was permitted to contain a source   routing list of hosts and the destination mailbox; however,   contemporary SMTP clients SHOULD NOT utilize source routes (seeAppendix C).  Servers MUST be prepared to encounter a list of source   routes in the forward-path, but they SHOULD ignore the routes or MAY   decline to support the relaying they imply.  Similarly, servers MAY   decline to accept mail that is destined for other hosts or systems.   These restrictions make a server useless as a relay for clients that   do not support full SMTP functionality.  Consequently, restricted-   capability clients MUST NOT assume that any SMTP server on the   Internet can be used as their mail processing (relaying) site.  If a   RCPT command appears without a previous MAIL command, the server MUST   return a 503 "Bad sequence of commands" response.  The optional   <rcpt-parameters> are associated with negotiated SMTP service   extensions (seeSection 2.2).   Since it has been a common source of errors, it is worth noting that   spaces are not permitted on either side of the colon following FROM   in the MAIL command or TO in the RCPT command.  The syntax is exactly   as given above.   The third step in the procedure is the DATA command (or some   alternative specified in a service extension).      DATA <CRLF>   If accepted, the SMTP server returns a 354 Intermediate reply and   considers all succeeding lines up to but not including the end of   mail data indicator to be the message text.  When the end of text is   successfully received and stored, the SMTP-receiver sends a "250 OK"   reply.   Since the mail data is sent on the transmission channel, the end of   mail data must be indicated so that the command and reply dialog can   be resumed.  SMTP indicates the end of the mail data by sending a   line containing only a "." (period or full stop).  A transparency   procedure is used to prevent this from interfering with the user's   text (seeSection 4.5.2).   The end of mail data indicator also confirms the mail transaction and   tells the SMTP server to now process the stored recipients and mailKlensin                     Standards Track                    [Page 20]

RFC 5321                          SMTP                      October 2008   data.  If accepted, the SMTP server returns a "250 OK" reply.  The   DATA command can fail at only two points in the protocol exchange:   If there was no MAIL, or no RCPT, command, or all such commands were   rejected, the server MAY return a "command out of sequence" (503) or   "no valid recipients" (554) reply in response to the DATA command.   If one of those replies (or any other 5yz reply) is received, the   client MUST NOT send the message data; more generally, message data   MUST NOT be sent unless a 354 reply is received.   If the verb is initially accepted and the 354 reply issued, the DATA   command should fail only if the mail transaction was incomplete (for   example, no recipients), if resources were unavailable (including, of   course, the server unexpectedly becoming unavailable), or if the   server determines that the message should be rejected for policy or   other reasons.   However, in practice, some servers do not perform recipient   verification until after the message text is received.  These servers   SHOULD treat a failure for one or more recipients as a "subsequent   failure" and return a mail message as discussed inSection 6 and, in   particular, inSection 6.1.  Using a "550 mailbox not found" (or   equivalent) reply code after the data are accepted makes it difficult   or impossible for the client to determine which recipients failed.   When theRFC 822 format ([28], [4]) is being used, the mail data   include the header fields such as those named Date, Subject, To, Cc,   and From.  Server SMTP systems SHOULD NOT reject messages based on   perceived defects in theRFC 822 or MIME (RFC 2045 [21]) message   header section or message body.  In particular, they MUST NOT reject   messages in which the numbers of Resent-header fields do not match or   Resent-to appears without Resent-from and/or Resent-date.   Mail transaction commands MUST be used in the order discussed above.3.4.  Forwarding for Address Correction or Updating   Forwarding support is most often required to consolidate and simplify   addresses within, or relative to, some enterprise and less frequently   to establish addresses to link a person's prior address with a   current one.  Silent forwarding of messages (without server   notification to the sender), for security or non-disclosure purposes,   is common in the contemporary Internet.   In both the enterprise and the "new address" cases, information   hiding (and sometimes security) considerations argue against exposure   of the "final" address through the SMTP protocol as a side effect of   the forwarding activity.  This may be especially important when theKlensin                     Standards Track                    [Page 21]

RFC 5321                          SMTP                      October 2008   final address may not even be reachable by the sender.  Consequently,   the "forwarding" mechanisms described inSection 3.2 of RFC 821, and   especially the 251 (corrected destination) and 551 reply codes from   RCPT must be evaluated carefully by implementers and, when they are   available, by those configuring systems (see alsoSection 7.4).   In particular:   o  Servers MAY forward messages when they are aware of an address      change.  When they do so, they MAY either provide address-updating      information with a 251 code, or may forward "silently" and return      a 250 code.  However, if a 251 code is used, they MUST NOT assume      that the client will actually update address information or even      return that information to the user.   Alternately,   o  Servers MAY reject messages or return them as non-deliverable when      they cannot be delivered precisely as addressed.  When they do so,      they MAY either provide address-updating information with a 551      code, or may reject the message as undeliverable with a 550 code      and no address-specific information.  However, if a 551 code is      used, they MUST NOT assume that the client will actually update      address information or even return that information to the user.   SMTP server implementations that support the 251 and/or 551 reply   codes SHOULD provide configuration mechanisms so that sites that   conclude that they would undesirably disclose information can disable   or restrict their use.3.5.  Commands for Debugging Addresses3.5.1.  Overview   SMTP provides commands to verify a user name or obtain the content of   a mailing list.  This is done with the VRFY and EXPN commands, which   have character string arguments.  Implementations SHOULD support VRFY   and EXPN (however, seeSection 3.5.2 andSection 7.3).   For the VRFY command, the string is a user name or a user name and   domain (see below).  If a normal (i.e., 250) response is returned,   the response MAY include the full name of the user and MUST include   the mailbox of the user.  It MUST be in either of the following   forms:      User Name <local-part@domain>      local-part@domainKlensin                     Standards Track                    [Page 22]

RFC 5321                          SMTP                      October 2008   When a name that is the argument to VRFY could identify more than one   mailbox, the server MAY either note the ambiguity or identify the   alternatives.  In other words, any of the following are legitimate   responses to VRFY:      553 User ambiguous   or      553- Ambiguous; Possibilities are      553-Joe Smith <jsmith@foo.com>      553-Harry Smith <hsmith@foo.com>      553 Melvin Smith <dweep@foo.com>   or      553-Ambiguous; Possibilities      553- <jsmith@foo.com>      553- <hsmith@foo.com>      553 <dweep@foo.com>   Under normal circumstances, a client receiving a 553 reply would be   expected to expose the result to the user.  Use of exactly the forms   given, and the "user ambiguous" or "ambiguous" keywords, possibly   supplemented by extended reply codes, such as those described inRFC3463 [25], will facilitate automated translation into other languages   as needed.  Of course, a client that was highly automated or that was   operating in another language than English might choose to try to   translate the response to return some other indication to the user   than the literal text of the reply, or to take some automated action   such as consulting a directory service for additional information   before reporting to the user.   For the EXPN command, the string identifies a mailing list, and the   successful (i.e., 250) multiline response MAY include the full name   of the users and MUST give the mailboxes on the mailing list.   In some hosts, the distinction between a mailing list and an alias   for a single mailbox is a bit fuzzy, since a common data structure   may hold both types of entries, and it is possible to have mailing   lists containing only one mailbox.  If a request is made to apply   VRFY to a mailing list, a positive response MAY be given if a message   so addressed would be delivered to everyone on the list, otherwise an   error SHOULD be reported (e.g., "550 That is a mailing list, not a   user" or "252 Unable to verify members of mailing list").  If a   request is made to expand a user name, the server MAY return aKlensin                     Standards Track                    [Page 23]

RFC 5321                          SMTP                      October 2008   positive response consisting of a list containing one name, or an   error MAY be reported (e.g., "550 That is a user name, not a mailing   list").   In the case of a successful multiline reply (normal for EXPN),   exactly one mailbox is to be specified on each line of the reply.   The case of an ambiguous request is discussed above.   "User name" is a fuzzy term and has been used deliberately.  An   implementation of the VRFY or EXPN commands MUST include at least   recognition of local mailboxes as "user names".  However, since   current Internet practice often results in a single host handling   mail for multiple domains, hosts, especially hosts that provide this   functionality, SHOULD accept the "local-part@domain" form as a "user   name"; hosts MAY also choose to recognize other strings as "user   names".   The case of expanding a mailbox list requires a multiline reply, such   as:      C: EXPN Example-People      S: 250-Jon Postel <Postel@isi.edu>      S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>      S: 250 Sam Q. Smith <SQSmith@specific.generic.com>   or      C: EXPN Executive-Washroom-List      S: 550 Access Denied to You.   The character string arguments of the VRFY and EXPN commands cannot   be further restricted due to the variety of implementations of the   user name and mailbox list concepts.  On some systems, it may be   appropriate for the argument of the EXPN command to be a file name   for a file containing a mailing list, but again there are a variety   of file naming conventions in the Internet.  Similarly, historical   variations in what is returned by these commands are such that the   response SHOULD be interpreted very carefully, if at all, and SHOULD   generally only be used for diagnostic purposes.3.5.2.  VRFY Normal Response   When normal (2yz or 551) responses are returned from a VRFY or EXPN   request, the reply MUST include the <Mailbox> name using a   "<local-part@domain>" construction, where "domain" is a fully-   qualified domain name.  In circumstances exceptional enough to   justify violating the intent of this specification, free-form text   MAY be returned.  In order to facilitate parsing by both computersKlensin                     Standards Track                    [Page 24]

RFC 5321                          SMTP                      October 2008   and people, addresses SHOULD appear in pointed brackets.  When   addresses, rather than free-form debugging information, are returned,   EXPN and VRFY MUST return only valid domain addresses that are usable   in SMTP RCPT commands.  Consequently, if an address implies delivery   to a program or other system, the mailbox name used to reach that   target MUST be given.  Paths (explicit source routes) MUST NOT be   returned by VRFY or EXPN.   Server implementations SHOULD support both VRFY and EXPN.  For   security reasons, implementations MAY provide local installations a   way to disable either or both of these commands through configuration   options or the equivalent (seeSection 7.3).  When these commands are   supported, they are not required to work across relays when relaying   is supported.  Since they were both optional inRFC 821, but VRFY was   made mandatory inRFC 1123 [3], if EXPN is supported, it MUST be   listed as a service extension in an EHLO response.  VRFY MAY be   listed as a convenience but, since support for it is required, SMTP   clients are not required to check for its presence on the extension   list before using it.3.5.3.  Meaning of VRFY or EXPN Success Response   A server MUST NOT return a 250 code in response to a VRFY or EXPN   command unless it has actually verified the address.  In particular,   a server MUST NOT return 250 if all it has done is to verify that the   syntax given is valid.  In that case, 502 (Command not implemented)   or 500 (Syntax error, command unrecognized) SHOULD be returned.  As   stated elsewhere, implementation (in the sense of actually validating   addresses and returning information) of VRFY and EXPN are strongly   recommended.  Hence, implementations that return 500 or 502 for VRFY   are not in full compliance with this specification.   There may be circumstances where an address appears to be valid but   cannot reasonably be verified in real time, particularly when a   server is acting as a mail exchanger for another server or domain.   "Apparent validity", in this case, would normally involve at least   syntax checking and might involve verification that any domains   specified were ones to which the host expected to be able to relay   mail.  In these situations, reply code 252 SHOULD be returned.  These   cases parallel the discussion of RCPT verification inSection 2.1.   Similarly, the discussion inSection 3.4 applies to the use of reply   codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are   recognized but that would be forwarded or rejected were mail received   for them.  Implementations generally SHOULD be more aggressive about   address verification in the case of VRFY than in the case of RCPT,   even if it takes a little longer to do so.Klensin                     Standards Track                    [Page 25]

RFC 5321                          SMTP                      October 20083.5.4.  Semantics and Applications of EXPN   EXPN is often very useful in debugging and understanding problems   with mailing lists and multiple-target-address aliases.  Some systems   have attempted to use source expansion of mailing lists as a means of   eliminating duplicates.  The propagation of aliasing systems with   mail on the Internet for hosts (typically with MX and CNAME DNS   records), for mailboxes (various types of local host aliases), and in   various proxying arrangements has made it nearly impossible for these   strategies to work consistently, and mail systems SHOULD NOT attempt   them.3.6.  Relaying and Mail Routing3.6.1.  Source Routes and Relaying   In general, the availability of Mail eXchanger records in the domain   name system (RFC 1035 [2],RFC 974 [12]) makes the use of explicit   source routes in the Internet mail system unnecessary.  Many   historical problems with the interpretation of explicit source routes   have made their use undesirable.  SMTP clients SHOULD NOT generate   explicit source routes except under unusual circumstances.  SMTP   servers MAY decline to act as mail relays or to accept addresses that   specify source routes.  When route information is encountered, SMTP   servers MAY ignore the route information and simply send to the final   destination specified as the last element in the route and SHOULD do   so.  There has been an invalid practice of using names that do not   appear in the DNS as destination names, with the senders counting on   the intermediate hosts specified in source routing to resolve any   problems.  If source routes are stripped, this practice will cause   failures.  This is one of several reasons why SMTP clients MUST NOT   generate invalid source routes or depend on serial resolution of   names.   When source routes are not used, the process described inRFC 821 for   constructing a reverse-path from the forward-path is not applicable   and the reverse-path at the time of delivery will simply be the   address that appeared in the MAIL command.3.6.2.  Mail eXchange Records and Relaying   A relay SMTP server is usually the target of a DNS MX record that   designates it, rather than the final delivery system.  The relay   server may accept or reject the task of relaying the mail in the same   way it accepts or rejects mail for a local user.  If it accepts the   task, it then becomes an SMTP client, establishes a transmission   channel to the next SMTP server specified in the DNS (according to   the rules inSection 5), and sends it the mail.  If it declines toKlensin                     Standards Track                    [Page 26]

RFC 5321                          SMTP                      October 2008   relay mail to a particular address for policy reasons, a 550 response   SHOULD be returned.   This specification does not deal with the verification of return   paths for use in delivery notifications.  Recent work, such as that   on SPF [29] and DKIM [30] [31], has been done to provide ways to   ascertain that an address is valid or belongs to the person who   actually sent the message.  A server MAY attempt to verify the return   path before using its address for delivery notifications, but methods   of doing so are not defined here nor is any particular method   recommended at this time.3.6.3.  Message Submission Servers as Relays   Many mail-sending clients exist, especially in conjunction with   facilities that receive mail via POP3 or IMAP, that have limited   capability to support some of the requirements of this specification,   such as the ability to queue messages for subsequent delivery   attempts.  For these clients, it is common practice to make private   arrangements to send all messages to a single server for processing   and subsequent distribution.  SMTP, as specified here, is not ideally   suited for this role.  A standardized mail submission protocol has   been developed that is gradually superseding practices based on SMTP   (seeRFC 4409 [18]).  In any event, because these arrangements are   private and fall outside the scope of this specification, they are   not described here.   It is important to note that MX records can point to SMTP servers   that act as gateways into other environments, not just SMTP relays   and final delivery systems; see Sections3.7 and5.   If an SMTP server has accepted the task of relaying the mail and   later finds that the destination is incorrect or that the mail cannot   be delivered for some other reason, then it MUST construct an   "undeliverable mail" notification message and send it to the   originator of the undeliverable mail (as indicated by the reverse-   path).  Formats specified for non-delivery reports by other standards   (see, for example,RFC 3461 [32] andRFC 3464 [33]) SHOULD be used if   possible.   This notification message must be from the SMTP server at the relay   host or the host that first determines that delivery cannot be   accomplished.  Of course, SMTP servers MUST NOT send notification   messages about problems transporting notification messages.  One way   to prevent loops in error reporting is to specify a null reverse-path   in the MAIL command of a notification message.  When such a message   is transmitted, the reverse-path MUST be set to null (seeKlensin                     Standards Track                    [Page 27]

RFC 5321                          SMTP                      October 2008Section 4.5.5 for additional discussion).  A MAIL command with a null   reverse-path appears as follows:      MAIL FROM:<>   As discussed inSection 6.4, a relay SMTP has no need to inspect or   act upon the header section or body of the message data and MUST NOT   do so except to add its own "Received:" header field (Section 4.4)   and, optionally, to attempt to detect looping in the mail system (seeSection 6.3).  Of course, this prohibition also applies to any   modifications of these header fields or text (see alsoSection 7.9).3.7.  Mail Gatewaying   While the relay function discussed above operates within the Internet   SMTP transport service environment, MX records or various forms of   explicit routing may require that an intermediate SMTP server perform   a translation function between one transport service and another.  As   discussed inSection 2.3.10, when such a system is at the boundary   between two transport service environments, we refer to it as a   "gateway" or "gateway SMTP".   Gatewaying mail between different mail environments, such as   different mail formats and protocols, is complex and does not easily   yield to standardization.  However, some general requirements may be   given for a gateway between the Internet and another mail   environment.3.7.1.  Header Fields in Gatewaying   Header fields MAY be rewritten when necessary as messages are   gatewayed across mail environment boundaries.  This may involve   inspecting the message body or interpreting the local-part of the   destination address in spite of the prohibitions inSection 6.4.   Other mail systems gatewayed to the Internet often use a subset of   theRFC 822 header section or provide similar functionality with a   different syntax, but some of these mail systems do not have an   equivalent to the SMTP envelope.  Therefore, when a message leaves   the Internet environment, it may be necessary to fold the SMTP   envelope information into the message header section.  A possible   solution would be to create new header fields to carry the envelope   information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this   would require changes in mail programs in foreign environments and   might risk disclosure of private information (seeSection 7.2).Klensin                     Standards Track                    [Page 28]

RFC 5321                          SMTP                      October 20083.7.2.  Received Lines in Gatewaying   When forwarding a message into or out of the Internet environment, a   gateway MUST prepend a Received: line, but it MUST NOT alter in any   way a Received: line that is already in the header section.   "Received:" header fields of messages originating from other   environments may not conform exactly to this specification.  However,   the most important use of Received: lines is for debugging mail   faults, and this debugging can be severely hampered by well-meaning   gateways that try to "fix" a Received: line.  As another consequence   of trace header fields arising in non-SMTP environments, receiving   systems MUST NOT reject mail based on the format of a trace header   field and SHOULD be extremely robust in the light of unexpected   information or formats in those header fields.   The gateway SHOULD indicate the environment and protocol in the "via"   clauses of Received header field(s) that it supplies.3.7.3.  Addresses in Gatewaying   From the Internet side, the gateway SHOULD accept all valid address   formats in SMTP commands and in theRFC 822 header section, and all   validRFC 822 messages.  Addresses and header fields generated by   gateways MUST conform to applicable standards (including this one andRFC 5322 [4]).  Gateways are, of course, subject to the same rules   for handling source routes as those described for other SMTP systems   inSection 3.3.3.7.4.  Other Header Fields in Gatewaying   The gateway MUST ensure that all header fields of a message that it   forwards into the Internet mail environment meet the requirements for   Internet mail.  In particular, all addresses in "From:", "To:",   "Cc:", etc., header fields MUST be transformed (if necessary) to   satisfy the standard header syntax ofRFC 5322 [4], MUST reference   only fully-qualified domain names, and MUST be effective and useful   for sending replies.  The translation algorithm used to convert mail   from the Internet protocols to another environment's protocol SHOULD   ensure that error messages from the foreign mail environment are   delivered to the reverse-path from the SMTP envelope, not to an   address in the "From:", "Sender:", or similar header fields of the   message.Klensin                     Standards Track                    [Page 29]

RFC 5321                          SMTP                      October 20083.7.5.  Envelopes in Gatewaying   Similarly, when forwarding a message from another environment into   the Internet, the gateway SHOULD set the envelope return path in   accordance with an error message return address, if supplied by the   foreign environment.  If the foreign environment has no equivalent   concept, the gateway must select and use a best approximation, with   the message originator's address as the default of last resort.3.8.  Terminating Sessions and Connections   An SMTP connection is terminated when the client sends a QUIT   command.  The server responds with a positive reply code, after which   it closes the connection.   An SMTP server MUST NOT intentionally close the connection under   normal operational circumstances (seeSection 7.8) except:   o  After receiving a QUIT command and responding with a 221 reply.   o  After detecting the need to shut down the SMTP service and      returning a 421 response code.  This response code can be issued      after the server receives any command or, if necessary,      asynchronously from command receipt (on the assumption that the      client will receive it after the next command is issued).   o  After a timeout, as specified inSection 4.5.3.2, occurs waiting      for the client to send a command or data.   In particular, a server that closes connections in response to   commands that are not understood is in violation of this   specification.  Servers are expected to be tolerant of unknown   commands, issuing a 500 reply and awaiting further instructions from   the client.   An SMTP server that is forcibly shut down via external means SHOULD   attempt to send a line containing a 421 response code to the SMTP   client before exiting.  The SMTP client will normally read the 421   response code after sending its next command.   SMTP clients that experience a connection close, reset, or other   communications failure due to circumstances not under their control   (in violation of the intent of this specification but sometimes   unavoidable) SHOULD, to maintain the robustness of the mail system,   treat the mail transaction as if a 451 response had been received and   act accordingly.Klensin                     Standards Track                    [Page 30]

RFC 5321                          SMTP                      October 20083.9.  Mailing Lists and Aliases   An SMTP-capable host SHOULD support both the alias and the list   models of address expansion for multiple delivery.  When a message is   delivered or forwarded to each address of an expanded list form, the   return address in the envelope ("MAIL FROM:") MUST be changed to be   the address of a person or other entity who administers the list.   However, in this case, the message header section (RFC 5322 [4]) MUST   be left unchanged; in particular, the "From" field of the header   section is unaffected.   An important mail facility is a mechanism for multi-destination   delivery of a single message, by transforming (or "expanding" or   "exploding") a pseudo-mailbox address into a list of destination   mailbox addresses.  When a message is sent to such a pseudo-mailbox   (sometimes called an "exploder"), copies are forwarded or   redistributed to each mailbox in the expanded list.  Servers SHOULD   simply utilize the addresses on the list; application of heuristics   or other matching rules to eliminate some addresses, such as that of   the originator, is strongly discouraged.  We classify such a pseudo-   mailbox as an "alias" or a "list", depending upon the expansion   rules.3.9.1.  Alias   To expand an alias, the recipient mailer simply replaces the pseudo-   mailbox address in the envelope with each of the expanded addresses   in turn; the rest of the envelope and the message body are left   unchanged.  The message is then delivered or forwarded to each   expanded address.3.9.2.  List   A mailing list may be said to operate by "redistribution" rather than   by "forwarding".  To expand a list, the recipient mailer replaces the   pseudo-mailbox address in the envelope with each of the expanded   addresses in turn.  The return (backward-pointing) address in the   envelope is changed so that all error messages generated by the final   deliveries will be returned to a list administrator, not to the   message originator, who generally has no control over the contents of   the list and will typically find error messages annoying.  Note that   the key difference between handling aliases (Section 3.9.1) and   forwarding (this subsection) is the change to the backward-pointing   address in this case.  When a list constrains its processing to the   very limited set of modifications and actions described here, it is   attempting to emulate an MTA; such lists can be treated as a   continuation in email transit.Klensin                     Standards Track                    [Page 31]

RFC 5321                          SMTP                      October 2008   There exist mailing lists that perform additional, sometimes   extensive, modifications to a message and its envelope.  Such mailing   lists need to be viewed as full MUAs, which accept a delivery and   post a new message.4.  The SMTP Specifications4.1.  SMTP Commands4.1.1.  Command Semantics and Syntax   The SMTP commands define the mail transfer or the mail system   function requested by the user.  SMTP commands are character strings   terminated by <CRLF>.  The commands themselves are alphabetic   characters terminated by <SP> if parameters follow and <CRLF>   otherwise.  (In the interest of improved interoperability, SMTP   receivers SHOULD tolerate trailing white space before the terminating   <CRLF>.)  The syntax of the local part of a mailbox MUST conform to   receiver site conventions and the syntax specified inSection 4.1.2.   The SMTP commands are discussed below.  The SMTP replies are   discussed inSection 4.2.   A mail transaction involves several data objects that are   communicated as arguments to different commands.  The reverse-path is   the argument of the MAIL command, the forward-path is the argument of   the RCPT command, and the mail data is the argument of the DATA   command.  These arguments or data objects must be transmitted and   held, pending the confirmation communicated by the end of mail data   indication that finalizes the transaction.  The model for this is   that distinct buffers are provided to hold the types of data objects;   that is, there is a reverse-path buffer, a forward-path buffer, and a   mail data buffer.  Specific commands cause information to be appended   to a specific buffer, or cause one or more buffers to be cleared.   Several commands (RSET, DATA, QUIT) are specified as not permitting   parameters.  In the absence of specific extensions offered by the   server and accepted by the client, clients MUST NOT send such   parameters and servers SHOULD reject commands containing them as   having invalid syntax.4.1.1.1.  Extended HELLO (EHLO) or HELLO (HELO)   These commands are used to identify the SMTP client to the SMTP   server.  The argument clause contains the fully-qualified domain name   of the SMTP client, if one is available.  In situations in which the   SMTP client system does not have a meaningful domain name (e.g., when   its address is dynamically allocated and no reverse mapping record isKlensin                     Standards Track                    [Page 32]

RFC 5321                          SMTP                      October 2008   available), the client SHOULD send an address literal (seeSection 4.1.3).RFC 2821, and some earlier informal practices, encouraged following   the literal by information that would help to identify the client   system.  That convention was not widely supported, and many SMTP   servers considered it an error.  In the interest of interoperability,   it is probably wise for servers to be prepared for this string to   occur, but SMTP clients SHOULD NOT send it.   The SMTP server identifies itself to the SMTP client in the   connection greeting reply and in the response to this command.   A client SMTP SHOULD start an SMTP session by issuing the EHLO   command.  If the SMTP server supports the SMTP service extensions, it   will give a successful response, a failure response, or an error   response.  If the SMTP server, in violation of this specification,   does not support any SMTP service extensions, it will generate an   error response.  Older client SMTP systems MAY, as discussed above,   use HELO (as specified inRFC 821) instead of EHLO, and servers MUST   support the HELO command and reply properly to it.  In any event, a   client MUST issue HELO or EHLO before starting a mail transaction.   These commands, and a "250 OK" reply to one of them, confirm that   both the SMTP client and the SMTP server are in the initial state,   that is, there is no transaction in progress and all state tables and   buffers are cleared.   Syntax:   ehlo           = "EHLO" SP ( Domain / address-literal ) CRLF   helo           = "HELO" SP Domain CRLF   Normally, the response to EHLO will be a multiline reply.  Each line   of the response contains a keyword and, optionally, one or more   parameters.  Following the normal syntax for multiline replies, these   keywords follow the code (250) and a hyphen for all but the last   line, and the code and a space for the last line.  The syntax for a   positive response, using the ABNF notation and terminal symbols ofRFC 5234 [7], is:   ehlo-ok-rsp    = ( "250" SP Domain [ SP ehlo-greet ] CRLF )                    / ( "250-" Domain [ SP ehlo-greet ] CRLF                    *( "250-" ehlo-line CRLF )                    "250" SP ehlo-line CRLF )Klensin                     Standards Track                    [Page 33]

RFC 5321                          SMTP                      October 2008   ehlo-greet     = 1*(%d0-9 / %d11-12 / %d14-127)                    ; string of any characters other than CR or LF   ehlo-line      = ehlo-keyword *( SP ehlo-param )   ehlo-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")                    ; additional syntax of ehlo-params depends on                    ; ehlo-keyword   ehlo-param     = 1*(%d33-126)                    ; any CHAR excluding <SP> and all                    ; control characters (US-ASCII 0-31 and 127                    ; inclusive)   Although EHLO keywords may be specified in upper, lower, or mixed   case, they MUST always be recognized and processed in a case-   insensitive manner.  This is simply an extension of practices   specified inRFC 821 andSection 2.4.   The EHLO response MUST contain keywords (and associated parameters if   required) for all commands not listed as "required" inSection 4.5.1   excepting only private-use commands as described inSection 4.1.5.   Private-use commands MAY be listed.4.1.1.2.  MAIL (MAIL)   This command is used to initiate a mail transaction in which the mail   data is delivered to an SMTP server that may, in turn, deliver it to   one or more mailboxes or pass it on to another system (possibly using   SMTP).  The argument clause contains a reverse-path and may contain   optional parameters.  In general, the MAIL command may be sent only   when no mail transaction is in progress, seeSection 4.1.4.   The reverse-path consists of the sender mailbox.  Historically, that   mailbox might optionally have been preceded by a list of hosts, but   that behavior is now deprecated (seeAppendix C).  In some types of   reporting messages for which a reply is likely to cause a mail loop   (for example, mail delivery and non-delivery notifications), the   reverse-path may be null (seeSection 3.6).   This command clears the reverse-path buffer, the forward-path buffer,   and the mail data buffer, and it inserts the reverse-path information   from its argument clause into the reverse-path buffer.   If service extensions were negotiated, the MAIL command may also   carry parameters associated with a particular service extension.Klensin                     Standards Track                    [Page 34]

RFC 5321                          SMTP                      October 2008   Syntax:   mail = "MAIL FROM:" Reverse-path                                       [SP Mail-parameters] CRLF4.1.1.3.  RECIPIENT (RCPT)   This command is used to identify an individual recipient of the mail   data; multiple recipients are specified by multiple uses of this   command.  The argument clause contains a forward-path and may contain   optional parameters.   The forward-path normally consists of the required destination   mailbox.  Sending systems SHOULD NOT generate the optional list of   hosts known as a source route.  Receiving systems MUST recognize   source route syntax but SHOULD strip off the source route   specification and utilize the domain name associated with the mailbox   as if the source route had not been provided.   Similarly, relay hosts SHOULD strip or ignore source routes, and   names MUST NOT be copied into the reverse-path.  When mail reaches   its ultimate destination (the forward-path contains only a   destination mailbox), the SMTP server inserts it into the destination   mailbox in accordance with its host mail conventions.   This command appends its forward-path argument to the forward-path   buffer; it does not change the reverse-path buffer nor the mail data   buffer.   For example, mail received at relay host xyz.com with envelope   commands      MAIL FROM:<userx@y.foo.org>      RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>   will normally be sent directly on to host d.bar.org with envelope   commands      MAIL FROM:<userx@y.foo.org>      RCPT TO:<userc@d.bar.org>   As provided inAppendix C, xyz.com MAY also choose to relay the   message to hosta.int, using the envelope commands      MAIL FROM:<userx@y.foo.org>      RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>Klensin                     Standards Track                    [Page 35]

RFC 5321                          SMTP                      October 2008   or to jkl.org, using the envelope commands      MAIL FROM:<userx@y.foo.org>      RCPT TO:<@jkl.org:userc@d.bar.org>   Attempting to use relaying this way is now strongly discouraged.   Since hosts are not required to relay mail at all, xyz.com MAY also   reject the message entirely when the RCPT command is received, using   a 550 code (since this is a "policy reason").   If service extensions were negotiated, the RCPT command may also   carry parameters associated with a particular service extension   offered by the server.  The client MUST NOT transmit parameters other   than those associated with a service extension offered by the server   in its EHLO response.   Syntax:      rcpt = "RCPT TO:" ( "<Postmaster@" Domain ">" / "<Postmaster>" /                  Forward-path ) [SP Rcpt-parameters] CRLF                  Note that, in a departure from the usual rules for                  local-parts, the "Postmaster" string shown above is                  treated as case-insensitive.4.1.1.4.  DATA (DATA)   The receiver normally sends a 354 response to DATA, and then treats   the lines (strings ending in <CRLF> sequences, as described inSection 2.3.7) following the command as mail data from the sender.   This command causes the mail data to be appended to the mail data   buffer.  The mail data may contain any of the 128 ASCII character   codes, although experience has indicated that use of control   characters other than SP, HT, CR, and LF may cause problems and   SHOULD be avoided when possible.   The mail data are terminated by a line containing only a period, that   is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is   actually the terminator of the previous line (seeSection 4.5.2).   This is the end of mail data indication.  The first <CRLF> of this   terminating sequence is also the <CRLF> that ends the final line of   the data (message text) or, if there was no mail data, ends the DATA   command itself (the "no mail data" case does not conform to this   specification since it would require that neither the trace header   fields required by this specification nor the message header section   required byRFC 5322 [4] be transmitted).  An extra <CRLF> MUST NOT   be added, as that would cause an empty line to be added to the   message.  The only exception to this rule would arise if the messageKlensin                     Standards Track                    [Page 36]

RFC 5321                          SMTP                      October 2008   body were passed to the originating SMTP-sender with a final "line"   that did not end in <CRLF>; in that case, the originating SMTP system   MUST either reject the message as invalid or add <CRLF> in order to   have the receiving SMTP server recognize the "end of data" condition.   The custom of accepting lines ending only in <LF>, as a concession to   non-conforming behavior on the part of some UNIX systems, has proven   to cause more interoperability problems than it solves, and SMTP   server systems MUST NOT do this, even in the name of improved   robustness.  In particular, the sequence "<LF>.<LF>" (bare line   feeds, without carriage returns) MUST NOT be treated as equivalent to   <CRLF>.<CRLF> as the end of mail data indication.   Receipt of the end of mail data indication requires the server to   process the stored mail transaction information.  This processing   consumes the information in the reverse-path buffer, the forward-path   buffer, and the mail data buffer, and on the completion of this   command these buffers are cleared.  If the processing is successful,   the receiver MUST send an OK reply.  If the processing fails, the   receiver MUST send a failure reply.  The SMTP model does not allow   for partial failures at this point: either the message is accepted by   the server for delivery and a positive response is returned or it is   not accepted and a failure reply is returned.  In sending a positive   "250 OK" completion reply to the end of data indication, the receiver   takes full responsibility for the message (seeSection 6.1).  Errors   that are diagnosed subsequently MUST be reported in a mail message,   as discussed inSection 4.4.   When the SMTP server accepts a message either for relaying or for   final delivery, it inserts a trace record (also referred to   interchangeably as a "time stamp line" or "Received" line) at the top   of the mail data.  This trace record indicates the identity of the   host that sent the message, the identity of the host that received   the message (and is inserting this time stamp), and the date and time   the message was received.  Relayed messages will have multiple time   stamp lines.  Details for formation of these lines, including their   syntax, is specified inSection 4.4.   Additional discussion about the operation of the DATA command appears   inSection 3.3.   Syntax:      data = "DATA" CRLFKlensin                     Standards Track                    [Page 37]

RFC 5321                          SMTP                      October 20084.1.1.5.  RESET (RSET)   This command specifies that the current mail transaction will be   aborted.  Any stored sender, recipients, and mail data MUST be   discarded, and all buffers and state tables cleared.  The receiver   MUST send a "250 OK" reply to a RSET command with no arguments.  A   reset command may be issued by the client at any time.  It is   effectively equivalent to a NOOP (i.e., it has no effect) if issued   immediately after EHLO, before EHLO is issued in the session, after   an end of data indicator has been sent and acknowledged, or   immediately before a QUIT.  An SMTP server MUST NOT close the   connection as the result of receiving a RSET; that action is reserved   for QUIT (seeSection 4.1.1.10).   Since EHLO implies some additional processing and response by the   server, RSET will normally be more efficient than reissuing that   command, even though the formal semantics are the same.   There are circumstances, contrary to the intent of this   specification, in which an SMTP server may receive an indication that   the underlying TCP connection has been closed or reset.  To preserve   the robustness of the mail system, SMTP servers SHOULD be prepared   for this condition and SHOULD treat it as if a QUIT had been received   before the connection disappeared.   Syntax:      rset = "RSET" CRLF4.1.1.6.  VERIFY (VRFY)   This command asks the receiver to confirm that the argument   identifies a user or mailbox.  If it is a user name, information is   returned as specified inSection 3.5.   This command has no effect on the reverse-path buffer, the forward-   path buffer, or the mail data buffer.   Syntax:      vrfy = "VRFY" SP String CRLFKlensin                     Standards Track                    [Page 38]

RFC 5321                          SMTP                      October 20084.1.1.7.  EXPAND (EXPN)   This command asks the receiver to confirm that the argument   identifies a mailing list, and if so, to return the membership of   that list.  If the command is successful, a reply is returned   containing information as described inSection 3.5.  This reply will   have multiple lines except in the trivial case of a one-member list.   This command has no effect on the reverse-path buffer, the forward-   path buffer, or the mail data buffer, and it may be issued at any   time.   Syntax:      expn = "EXPN" SP String CRLF4.1.1.8.  HELP (HELP)   This command causes the server to send helpful information to the   client.  The command MAY take an argument (e.g., any command name)   and return more specific information as a response.   This command has no effect on the reverse-path buffer, the forward-   path buffer, or the mail data buffer, and it may be issued at any   time.   SMTP servers SHOULD support HELP without arguments and MAY support it   with arguments.   Syntax:      help = "HELP" [ SP String ] CRLFKlensin                     Standards Track                    [Page 39]

RFC 5321                          SMTP                      October 20084.1.1.9.  NOOP (NOOP)   This command does not affect any parameters or previously entered   commands.  It specifies no action other than that the receiver send a   "250 OK" reply.   This command has no effect on the reverse-path buffer, the forward-   path buffer, or the mail data buffer, and it may be issued at any   time.  If a parameter string is specified, servers SHOULD ignore it.   Syntax:      noop = "NOOP" [ SP String ] CRLF4.1.1.10.  QUIT (QUIT)   This command specifies that the receiver MUST send a "221 OK" reply,   and then close the transmission channel.   The receiver MUST NOT intentionally close the transmission channel   until it receives and replies to a QUIT command (even if there was an   error).  The sender MUST NOT intentionally close the transmission   channel until it sends a QUIT command, and it SHOULD wait until it   receives the reply (even if there was an error response to a previous   command).  If the connection is closed prematurely due to violations   of the above or system or network failure, the server MUST cancel any   pending transaction, but not undo any previously completed   transaction, and generally MUST act as if the command or transaction   in progress had received a temporary error (i.e., a 4yz response).   The QUIT command may be issued at any time.  Any current uncompleted   mail transaction will be aborted.   Syntax:      quit = "QUIT" CRLF4.1.1.11.  Mail-Parameter and Rcpt-Parameter Error Responses   If the server SMTP does not recognize or cannot implement one or more   of the parameters associated with a particular MAIL FROM or RCPT TO   command, it will return code 555.   If, for some reason, the server is temporarily unable to accommodate   one or more of the parameters associated with a MAIL FROM or RCPT TO   command, and if the definition of the specific parameter does not   mandate the use of another code, it should return code 455.Klensin                     Standards Track                    [Page 40]

RFC 5321                          SMTP                      October 2008   Errors specific to particular parameters and their values will be   specified in the parameter's defining RFC.4.1.2.  Command Argument Syntax   The syntax of the argument clauses of the above commands (using the   syntax specified inRFC 5234 [7] where applicable) is given below.   Some of the productions given below are used only in conjunction with   source routes as described inAppendix C.  Terminals not defined in   this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined   in the "core" syntax inSection 6 of RFC 5234 [7] or in the message   format syntax inRFC 5322 [4].   Reverse-path   = Path / "<>"   Forward-path   = Path   Path           = "<" [ A-d-l ":" ] Mailbox ">"   A-d-l          = At-domain *( "," At-domain )                  ; Note that this form, the so-called "source                  ; route", MUST BE accepted, SHOULD NOT be                  ; generated, and SHOULD be ignored.   At-domain      = "@" Domain   Mail-parameters  = esmtp-param *(SP esmtp-param)   Rcpt-parameters  = esmtp-param *(SP esmtp-param)   esmtp-param    = esmtp-keyword ["=" esmtp-value]   esmtp-keyword  = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")   esmtp-value    = 1*(%d33-60 / %d62-126)                  ; any CHAR excluding "=", SP, and control                  ; characters.  If this string is an email address,                  ; i.e., a Mailbox, then the "xtext" syntax [32]                  ; SHOULD be used.   Keyword        = Ldh-str   Argument       = Atom   Domain         = sub-domain *("." sub-domain)Klensin                     Standards Track                    [Page 41]

RFC 5321                          SMTP                      October 2008   sub-domain     = Let-dig [Ldh-str]   Let-dig        = ALPHA / DIGIT   Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig   address-literal  = "[" ( IPv4-address-literal /                    IPv6-address-literal /                    General-address-literal ) "]"                    ; SeeSection 4.1.3   Mailbox        = Local-part "@" ( Domain / address-literal )   Local-part     = Dot-string / Quoted-string                  ; MAY be case-sensitive   Dot-string     = Atom *("."  Atom)   Atom           = 1*atext   Quoted-string  = DQUOTE *QcontentSMTP DQUOTE   QcontentSMTP   = qtextSMTP / quoted-pairSMTP   quoted-pairSMTP  = %d92 %d32-126                    ; i.e., backslash followed by any ASCII                    ; graphic (including itself) or SPace   qtextSMTP      = %d32-33 / %d35-91 / %d93-126                  ; i.e., within a quoted string, any                  ; ASCII graphic or space is permitted                  ; without blackslash-quoting except                  ; double-quote and the backslash itself.   String         = Atom / Quoted-string   While the above definition for Local-part is relatively permissive,   for maximum interoperability, a host that expects to receive mail   SHOULD avoid defining mailboxes where the Local-part requires (or   uses) the Quoted-string form or where the Local-part is case-   sensitive.  For any purposes that require generating or comparing   Local-parts (e.g., to specific mailbox names), all quoted forms MUST   be treated as equivalent, and the sending system SHOULD transmit the   form that uses the minimum quoting possible.   Systems MUST NOT define mailboxes in such a way as to require the use   in SMTP of non-ASCII characters (octets with the high order bit setKlensin                     Standards Track                    [Page 42]

RFC 5321                          SMTP                      October 2008   to one) or ASCII "control characters" (decimal value 0-31 and 127).   These characters MUST NOT be used in MAIL or RCPT commands or other   commands that require mailbox names.   Note that the backslash, "\", is a quote character, which is used to   indicate that the next character is to be used literally (instead of   its normal interpretation).  For example, "Joe\,Smith" indicates a   single nine-character user name string with the comma being the   fourth character of that string.   To promote interoperability and consistent with long-standing   guidance about conservative use of the DNS in naming and applications   (e.g., seeSection 2.3.1 of the base DNS document,RFC 1035 [2]),   characters outside the set of alphabetic characters, digits, and   hyphen MUST NOT appear in domain name labels for SMTP clients or   servers.  In particular, the underscore character is not permitted.   SMTP servers that receive a command in which invalid character codes   have been employed, and for which there are no other reasons for   rejection, MUST reject that command with a 501 response (this rule,   like others, could be overridden by appropriate SMTP extensions).4.1.3.  Address Literals   Sometimes a host is not known to the domain name system and   communication (and, in particular, communication to report and repair   the error) is blocked.  To bypass this barrier, a special literal   form of the address is allowed as an alternative to a domain name.   For IPv4 addresses, this form uses four small decimal integers   separated by dots and enclosed by brackets such as [123.255.37.2],   which indicates an (IPv4) Internet Address in sequence-of-octets   form.  For IPv6 and other forms of addressing that might eventually   be standardized, the form consists of a standardized "tag" that   identifies the address syntax, a colon, and the address itself, in a   format specified as part of the relevant standards (i.e.,RFC 4291   [8] for IPv6).   Specifically:   IPv4-address-literal  = Snum 3("."  Snum)   IPv6-address-literal  = "IPv6:" IPv6-addr   General-address-literal  = Standardized-tag ":" 1*dcontent   Standardized-tag  = Ldh-str                     ; Standardized-tag MUST be specified in a                     ; Standards-Track RFC and registered with IANAKlensin                     Standards Track                    [Page 43]

RFC 5321                          SMTP                      October 2008   dcontent       = %d33-90 / ; Printable US-ASCII                  %d94-126 ; excl. "[", "\", "]"   Snum           = 1*3DIGIT                  ; representing a decimal integer                  ; value in the range 0 through 255   IPv6-addr      = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp   IPv6-hex       = 1*4HEXDIG   IPv6-full      = IPv6-hex 7(":" IPv6-hex)   IPv6-comp      = [IPv6-hex *5(":" IPv6-hex)] "::"                  [IPv6-hex *5(":" IPv6-hex)]                  ; The "::" represents at least 2 16-bit groups of                  ; zeros.  No more than 6 groups in addition to the                  ; "::" may be present.   IPv6v4-full    = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal   IPv6v4-comp    = [IPv6-hex *3(":" IPv6-hex)] "::"                  [IPv6-hex *3(":" IPv6-hex) ":"]                  IPv4-address-literal                  ; The "::" represents at least 2 16-bit groups of                  ; zeros.  No more than 4 groups in addition to the                  ; "::" and IPv4-address-literal may be present.4.1.4.  Order of Commands   There are restrictions on the order in which these commands may be   used.   A session that will contain mail transactions MUST first be   initialized by the use of the EHLO command.  An SMTP server SHOULD   accept commands for non-mail transactions (e.g., VRFY or EXPN)   without this initialization.   An EHLO command MAY be issued by a client later in the session.  If   it is issued after the session begins and the EHLO command is   acceptable to the SMTP server, the SMTP server MUST clear all buffers   and reset the state exactly as if a RSET command had been issued.  In   other words, the sequence of RSET followed immediately by EHLO is   redundant, but not harmful other than in the performance cost of   executing unnecessary commands.   If the EHLO command is not acceptable to the SMTP server, 501, 500,   502, or 550 failure replies MUST be returned as appropriate.  TheKlensin                     Standards Track                    [Page 44]

RFC 5321                          SMTP                      October 2008   SMTP server MUST stay in the same state after transmitting these   replies that it was in before the EHLO was received.   The SMTP client MUST, if possible, ensure that the domain parameter   to the EHLO command is a primary host name as specified for this   command inSection 2.3.5.  If this is not possible (e.g., when the   client's address is dynamically assigned and the client does not have   an obvious name), an address literal SHOULD be substituted for the   domain name.   An SMTP server MAY verify that the domain name argument in the EHLO   command actually corresponds to the IP address of the client.   However, if the verification fails, the server MUST NOT refuse to   accept a message on that basis.  Information captured in the   verification attempt is for logging and tracing purposes.  Note that   this prohibition applies to the matching of the parameter to its IP   address only; seeSection 7.9 for a more extensive discussion of   rejecting incoming connections or mail messages.   The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time   during a session, or without previously initializing a session.  SMTP   servers SHOULD process these normally (that is, not return a 503   code) even if no EHLO command has yet been received; clients SHOULD   open a session with EHLO before sending these commands.   If these rules are followed, the example inRFC 821 that shows "550   access denied to you" in response to an EXPN command is incorrect   unless an EHLO command precedes the EXPN or the denial of access is   based on the client's IP address or other authentication or   authorization-determining mechanisms.   The MAIL command (or the obsolete SEND, SOML, or SAML commands)   begins a mail transaction.  Once started, a mail transaction consists   of a transaction beginning command, one or more RCPT commands, and a   DATA command, in that order.  A mail transaction may be aborted by   the RSET, a new EHLO, or the QUIT command.  There may be zero or more   transactions in a session.  MAIL (or SEND, SOML, or SAML) MUST NOT be   sent if a mail transaction is already open, i.e., it should be sent   only if no mail transaction had been started in the session, or if   the previous one successfully concluded with a successful DATA   command, or if the previous one was aborted, e.g., with a RSET or new   EHLO.   If the transaction beginning command argument is not acceptable, a   501 failure reply MUST be returned and the SMTP server MUST stay in   the same state.  If the commands in a transaction are out of order to   the degree that they cannot be processed by the server, a 503 failureKlensin                     Standards Track                    [Page 45]

RFC 5321                          SMTP                      October 2008   reply MUST be returned and the SMTP server MUST stay in the same   state.   The last command in a session MUST be the QUIT command.  The QUIT   command SHOULD be used by the client SMTP to request connection   closure, even when no session opening command was sent and accepted.4.1.5.  Private-Use Commands   As specified inSection 2.2.2, commands starting in "X" may be used   by bilateral agreement between the client (sending) and server   (receiving) SMTP agents.  An SMTP server that does not recognize such   a command is expected to reply with "500 Command not recognized".  An   extended SMTP server MAY list the feature names associated with these   private commands in the response to the EHLO command.   Commands sent or accepted by SMTP systems that do not start with "X"   MUST conform to the requirements ofSection 2.2.2.4.2.  SMTP Replies   Replies to SMTP commands serve to ensure the synchronization of   requests and actions in the process of mail transfer and to guarantee   that the SMTP client always knows the state of the SMTP server.   Every command MUST generate exactly one reply.   The details of the command-reply sequence are described inSection 4.3.   An SMTP reply consists of a three digit number (transmitted as three   numeric characters) followed by some text unless specified otherwise   in this document.  The number is for use by automata to determine   what state to enter next; the text is for the human user.  The three   digits contain enough encoded information that the SMTP client need   not examine the text and may either discard it or pass it on to the   user, as appropriate.  Exceptions are as noted elsewhere in this   document.  In particular, the 220, 221, 251, 421, and 551 reply codes   are associated with message text that must be parsed and interpreted   by machines.  In the general case, the text may be receiver dependent   and context dependent, so there are likely to be varying texts for   each reply code.  A discussion of the theory of reply codes is given   inSection 4.2.1.  Formally, a reply is defined to be the sequence: a   three-digit code, <SP>, one line of text, and <CRLF>, or a multiline   reply (as defined in the same section).  Since, in violation of this   specification, the text is sometimes not sent, clients that do not   receive it SHOULD be prepared to process the code alone (with or   without a trailing space character).  Only the EHLO, EXPN, and HELP   commands are expected to result in multiline replies in normalKlensin                     Standards Track                    [Page 46]

RFC 5321                          SMTP                      October 2008   circumstances; however, multiline replies are allowed for any   command.   In ABNF, server responses are:   Greeting       = ( "220 " (Domain / address-literal)                  [ SP textstring ] CRLF ) /                  ( "220-" (Domain / address-literal)                  [ SP textstring ] CRLF                  *( "220-" [ textstring ] CRLF )                  "220" [ SP textstring ] CRLF )   textstring     = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII   Reply-line     = *( Reply-code "-" [ textstring ] CRLF )                  Reply-code [ SP textstring ] CRLF   Reply-code     = %x32-35 %x30-35 %x30-39   where "Greeting" appears only in the 220 response that announces that   the server is opening its part of the connection.  (Other possible   server responses upon connection follow the syntax of Reply-line.)   An SMTP server SHOULD send only the reply codes listed in this   document.  An SMTP server SHOULD use the text shown in the examples   whenever appropriate.   An SMTP client MUST determine its actions only by the reply code, not   by the text (except for the "change of address" 251 and 551 and, if   necessary, 220, 221, and 421 replies); in the general case, any text,   including no text at all (although senders SHOULD NOT send bare   codes), MUST be acceptable.  The space (blank) following the reply   code is considered part of the text.  Whenever possible, a receiver-   SMTP SHOULD test the first digit (severity indication) of the reply   code.   The list of codes that appears below MUST NOT be construed as   permanent.  While the addition of new codes should be a rare and   significant activity, with supplemental information in the textual   part of the response being preferred, new codes may be added as the   result of new Standards or Standards-Track specifications.   Consequently, a sender-SMTP MUST be prepared to handle codes not   specified in this document and MUST do so by interpreting the first   digit only.   In the absence of extensions negotiated with the client, SMTP servers   MUST NOT send reply codes whose first digits are other than 2, 3, 4,Klensin                     Standards Track                    [Page 47]

RFC 5321                          SMTP                      October 2008   or 5.  Clients that receive such out-of-range codes SHOULD normally   treat them as fatal errors and terminate the mail transaction.4.2.1.  Reply Code Severities and Theory   The three digits of the reply each have a special significance.  The   first digit denotes whether the response is good, bad, or incomplete.   An unsophisticated SMTP client, or one that receives an unexpected   code, will be able to determine its next action (proceed as planned,   redo, retrench, etc.) by examining this first digit.  An SMTP client   that wants to know approximately what kind of error occurred (e.g.,   mail system error, command syntax error) may examine the second   digit.  The third digit and any supplemental information that may be   present is reserved for the finest gradation of information.   There are four values for the first digit of the reply code:   2yz  Positive Completion reply      The requested action has been successfully completed.  A new      request may be initiated.   3yz  Positive Intermediate reply      The command has been accepted, but the requested action is being      held in abeyance, pending receipt of further information.  The      SMTP client should send another command specifying this      information.  This reply is used in command sequence groups (i.e.,      in DATA).   4yz  Transient Negative Completion reply      The command was not accepted, and the requested action did not      occur.  However, the error condition is temporary, and the action      may be requested again.  The sender should return to the beginning      of the command sequence (if any).  It is difficult to assign a      meaning to "transient" when two different sites (receiver- and      sender-SMTP agents) must agree on the interpretation.  Each reply      in this category might have a different time value, but the SMTP      client SHOULD try again.  A rule of thumb to determine whether a      reply fits into the 4yz or the 5yz category (see below) is that      replies are 4yz if they can be successful if repeated without any      change in command form or in properties of the sender or receiver      (that is, the command is repeated identically and the receiver      does not put up a new implementation).   5yz  Permanent Negative Completion reply      The command was not accepted and the requested action did not      occur.  The SMTP client SHOULD NOT repeat the exact request (in      the same sequence).  Even some "permanent" error conditions can be      corrected, so the human user may want to direct the SMTP client toKlensin                     Standards Track                    [Page 48]

RFC 5321                          SMTP                      October 2008      reinitiate the command sequence by direct action at some point in      the future (e.g., after the spelling has been changed, or the user      has altered the account status).   It is worth noting that the file transfer protocol (FTP) [34] uses a   very similar code architecture and that the SMTP codes are based on   the FTP model.  However, SMTP uses a one-command, one-response model   (while FTP is asynchronous) and FTP's 1yz codes are not part of the   SMTP model.   The second digit encodes responses in specific categories:   x0z  Syntax: These replies refer to syntax errors, syntactically      correct commands that do not fit any functional category, and      unimplemented or superfluous commands.   x1z  Information: These are replies to requests for information, such      as status or help.   x2z  Connections: These are replies referring to the transmission      channel.   x3z  Unspecified.   x4z  Unspecified.   x5z  Mail system: These replies indicate the status of the receiver      mail system vis-a-vis the requested transfer or other mail system      action.   The third digit gives a finer gradation of meaning in each category   specified by the second digit.  The list of replies illustrates this.   Each reply text is recommended rather than mandatory, and may even   change according to the command with which it is associated.  On the   other hand, the reply codes must strictly follow the specifications   in this section.  Receiver implementations should not invent new   codes for slightly different situations from the ones described here,   but rather adapt codes already defined.   For example, a command such as NOOP, whose successful execution does   not offer the SMTP client any new information, will return a 250   reply.  The reply is 502 when the command requests an unimplemented   non-site-specific action.  A refinement of that is the 504 reply for   a command that is implemented, but that requests an unimplemented   parameter.Klensin                     Standards Track                    [Page 49]

RFC 5321                          SMTP                      October 2008   The reply text may be longer than a single line; in these cases the   complete text must be marked so the SMTP client knows when it can   stop reading the reply.  This requires a special format to indicate a   multiple line reply.   The format for multiline replies requires that every line, except the   last, begin with the reply code, followed immediately by a hyphen,   "-" (also known as minus), followed by text.  The last line will   begin with the reply code, followed immediately by <SP>, optionally   some text, and <CRLF>.  As noted above, servers SHOULD send the <SP>   if subsequent text is not sent, but clients MUST be prepared for it   to be omitted.   For example:      250-First line      250-Second line      250-234 Text beginning with numbers      250 The last line   In a multiline reply, the reply code on each of the lines MUST be the   same.  It is reasonable for the client to rely on this, so it can   make processing decisions based on the code in any line, assuming   that all others will be the same.  In a few cases, there is important   data for the client in the reply "text".  The client will be able to   identify these cases from the current context.4.2.2.  Reply Codes by Function Groups   500  Syntax error, command unrecognized (This may include errors such      as command line too long)   501  Syntax error in parameters or arguments   502  Command not implemented (seeSection 4.2.4)   503  Bad sequence of commands   504  Command parameter not implemented   211  System status, or system help reply   214  Help message (Information on how to use the receiver or the      meaning of a particular non-standard command; this reply is useful      only to the human user)Klensin                     Standards Track                    [Page 50]

RFC 5321                          SMTP                      October 2008   220  <domain> Service ready   221  <domain> Service closing transmission channel   421  <domain> Service not available, closing transmission channel      (This may be a reply to any command if the service knows it must      shut down)   250  Requested mail action okay, completed   251  User not local; will forward to <forward-path> (SeeSection 3.4)   252  Cannot VRFY user, but will accept message and attempt delivery      (SeeSection 3.5.3)   455  Server unable to accommodate parameters   555  MAIL FROM/RCPT TO parameters not recognized or not implemented   450  Requested mail action not taken: mailbox unavailable (e.g.,      mailbox busy or temporarily blocked for policy reasons)   550  Requested action not taken: mailbox unavailable (e.g., mailbox      not found, no access, or command rejected for policy reasons)   451  Requested action aborted: error in processing   551  User not local; please try <forward-path> (SeeSection 3.4)   452  Requested action not taken: insufficient system storage   552  Requested mail action aborted: exceeded storage allocation   553  Requested action not taken: mailbox name not allowed (e.g.,      mailbox syntax incorrect)   354  Start mail input; end with <CRLF>.<CRLF>   554  Transaction failed (Or, in the case of a connection-opening      response, "No SMTP service here")Klensin                     Standards Track                    [Page 51]

RFC 5321                          SMTP                      October 20084.2.3.  Reply Codes in Numeric Order   211  System status, or system help reply   214  Help message (Information on how to use the receiver or the      meaning of a particular non-standard command; this reply is useful      only to the human user)   220  <domain> Service ready   221  <domain> Service closing transmission channel   250  Requested mail action okay, completed   251  User not local; will forward to <forward-path> (SeeSection 3.4)   252  Cannot VRFY user, but will accept message and attempt delivery      (SeeSection 3.5.3)   354  Start mail input; end with <CRLF>.<CRLF>   421  <domain> Service not available, closing transmission channel      (This may be a reply to any command if the service knows it must      shut down)   450  Requested mail action not taken: mailbox unavailable (e.g.,      mailbox busy or temporarily blocked for policy reasons)   451  Requested action aborted: local error in processing   452  Requested action not taken: insufficient system storage   455  Server unable to accommodate parameters   500  Syntax error, command unrecognized (This may include errors such      as command line too long)   501  Syntax error in parameters or arguments   502  Command not implemented (seeSection 4.2.4)   503  Bad sequence of commands   504  Command parameter not implemented   550  Requested action not taken: mailbox unavailable (e.g., mailbox      not found, no access, or command rejected for policy reasons)Klensin                     Standards Track                    [Page 52]

RFC 5321                          SMTP                      October 2008   551  User not local; please try <forward-path> (SeeSection 3.4)   552  Requested mail action aborted: exceeded storage allocation   553  Requested action not taken: mailbox name not allowed (e.g.,      mailbox syntax incorrect)   554  Transaction failed (Or, in the case of a connection-opening      response, "No SMTP service here")   555  MAIL FROM/RCPT TO parameters not recognized or not implemented4.2.4.  Reply Code 502   Questions have been raised as to when reply code 502 (Command not   implemented) SHOULD be returned in preference to other codes. 502   SHOULD be used when the command is actually recognized by the SMTP   server, but not implemented.  If the command is not recognized, code   500 SHOULD be returned.  Extended SMTP systems MUST NOT list   capabilities in response to EHLO for which they will return 502 (or   500) replies.4.2.5.  Reply Codes after DATA and the Subsequent <CRLF>.<CRLF>   When an SMTP server returns a positive completion status (2yz code)   after the DATA command is completed with <CRLF>.<CRLF>, it accepts   responsibility for:   o  delivering the message (if the recipient mailbox exists), or   o  if attempts to deliver the message fail due to transient      conditions, retrying delivery some reasonable number of times at      intervals as specified inSection 4.5.4.   o  if attempts to deliver the message fail due to permanent      conditions, or if repeated attempts to deliver the message fail      due to transient conditions, returning appropriate notification to      the sender of the original message (using the address in the SMTP      MAIL command).   When an SMTP server returns a temporary error status (4yz) code after   the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make a   subsequent attempt to deliver that message.  The SMTP client retains   responsibility for the delivery of that message and may either return   it to the user or requeue it for a subsequent attempt (seeSection 4.5.4.1).Klensin                     Standards Track                    [Page 53]

RFC 5321                          SMTP                      October 2008   The user who originated the message SHOULD be able to interpret the   return of a transient failure status (by mail message or otherwise)   as a non-delivery indication, just as a permanent failure would be   interpreted.  If the client SMTP successfully handles these   conditions, the user will not receive such a reply.   When an SMTP server returns a permanent error status (5yz) code after   the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make   any subsequent attempt to deliver the message.  As with temporary   error status codes, the SMTP client retains responsibility for the   message, but SHOULD not again attempt delivery to the same server   without user review of the message and response and appropriate   intervention.4.3.  Sequencing of Commands and Replies4.3.1.  Sequencing Overview   The communication between the sender and receiver is an alternating   dialogue, controlled by the sender.  As such, the sender issues a   command and the receiver responds with a reply.  Unless other   arrangements are negotiated through service extensions, the sender   MUST wait for this response before sending further commands.  One   important reply is the connection greeting.  Normally, a receiver   will send a 220 "Service ready" reply when the connection is   completed.  The sender SHOULD wait for this greeting message before   sending any commands.   Note: all the greeting-type replies have the official name (the   fully-qualified primary domain name) of the server host as the first   word following the reply code.  Sometimes the host will have no   meaningful name.  SeeSection 4.1.3 for a discussion of alternatives   in these situations.   For example,      220 ISIF.USC.EDU Service ready   or      220 mail.example.com SuperSMTP v 6.1.2 Service ready   or      220 [10.0.0.1] Clueless host service ready   The table below lists alternative success and failure replies for   each command.  These SHOULD be strictly adhered to.  A receiver MAYKlensin                     Standards Track                    [Page 54]

RFC 5321                          SMTP                      October 2008   substitute text in the replies, but the meanings and actions implied   by the code numbers and by the specific command reply sequence MUST   be preserved.4.3.2.  Command-Reply Sequences   Each command is listed with its usual possible replies.  The prefixes   used before the possible replies are "I" for intermediate, "S" for   success, and "E" for error.  Since some servers may generate other   replies under special circumstances, and to allow for future   extension, SMTP clients SHOULD, when possible, interpret only the   first digit of the reply and MUST be prepared to deal with   unrecognized reply codes by interpreting the first digit only.   Unless extended using the mechanisms described inSection 2.2, SMTP   servers MUST NOT transmit reply codes to an SMTP client that are   other than three digits or that do not start in a digit between 2 and   5 inclusive.   These sequencing rules and, in principle, the codes themselves, can   be extended or modified by SMTP extensions offered by the server and   accepted (requested) by the client.  However, if the target is more   precise granularity in the codes, rather than codes for completely   new purposes, the system described inRFC 3463 [25] SHOULD be used in   preference to the invention of new codes.   In addition to the codes listed below, any SMTP command can return   any of the following codes if the corresponding unusual circumstances   are encountered:   500  For the "command line too long" case or if the command name was      not recognized.  Note that producing a "command not recognized"      error in response to the required subset of these commands is a      violation of this specification.  Similarly, producing a "command      too long" message for a command line shorter than 512 characters      would violate the provisions ofSection 4.5.3.1.4.   501  Syntax error in command or arguments.  In order to provide for      future extensions, commands that are specified in this document as      not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501      message if arguments are supplied in the absence of EHLO-      advertised extensions.   421  Service shutting down and closing transmission channelKlensin                     Standards Track                    [Page 55]

RFC 5321                          SMTP                      October 2008   Specific sequences are:      CONNECTION ESTABLISHMENT         S: 220         E: 554      EHLO or HELO         S: 250         E: 504 (a conforming implementation could return this code only         in fairly obscure cases), 550, 502 (permitted only with an old-         style server that does not support EHLO)      MAIL         S: 250         E: 552, 451, 452, 550, 553, 503, 455, 555      RCPT         S: 250, 251 (but seeSection 3.4 for discussion of 251 and 551)         E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555      DATA         I: 354 -> data -> S: 250                           E: 552, 554, 451, 452                           E: 450, 550 (rejections for policy reasons)         E: 503, 554      RSET         S: 250      VRFY         S: 250, 251, 252         E: 550, 551, 553, 502, 504      EXPN         S: 250, 252         E: 550, 500, 502, 504Klensin                     Standards Track                    [Page 56]

RFC 5321                          SMTP                      October 2008      HELP         S: 211, 214         E: 502, 504      NOOP         S: 250      QUIT         S: 2214.4.  Trace Information   When an SMTP server receives a message for delivery or further   processing, it MUST insert trace ("time stamp" or "Received")   information at the beginning of the message content, as discussed inSection 4.1.1.4.   This line MUST be structured as follows:   o  The FROM clause, which MUST be supplied in an SMTP environment,      SHOULD contain both (1) the name of the source host as presented      in the EHLO command and (2) an address literal containing the IP      address of the source, determined from the TCP connection.   o  The ID clause MAY contain an "@" as suggested inRFC 822, but this      is not required.   o  If the FOR clause appears, it MUST contain exactly one <path>      entry, even when multiple RCPT commands have been given.  Multiple      <path>s raise some security issues and have been deprecated, seeSection 7.2.   An Internet mail program MUST NOT change or delete a Received: line   that was previously added to the message header section.  SMTP   servers MUST prepend Received lines to messages; they MUST NOT change   the order of existing lines or insert Received lines in any other   location.   As the Internet grows, comparability of Received header fields is   important for detecting problems, especially slow relays.  SMTP   servers that create Received header fields SHOULD use explicit   offsets in the dates (e.g., -0800), rather than time zone names of   any type.  Local time (with an offset) SHOULD be used rather than UT   when feasible.  This formulation allows slightly more information   about local circumstances to be specified.  If UT is needed, theKlensin                     Standards Track                    [Page 57]

RFC 5321                          SMTP                      October 2008   receiver need merely do some simple arithmetic to convert the values.   Use of UT loses information about the time zone-location of the   server.  If it is desired to supply a time zone name, it SHOULD be   included in a comment.   When the delivery SMTP server makes the "final delivery" of a   message, it inserts a return-path line at the beginning of the mail   data.  This use of return-path is required; mail systems MUST support   it.  The return-path line preserves the information in the <reverse-   path> from the MAIL command.  Here, final delivery means the message   has left the SMTP environment.  Normally, this would mean it had been   delivered to the destination user or an associated mail drop, but in   some cases it may be further processed and transmitted by another   mail system.   It is possible for the mailbox in the return path to be different   from the actual sender's mailbox, for example, if error responses are   to be delivered to a special error handling mailbox rather than to   the message sender.  When mailing lists are involved, this   arrangement is common and useful as a means of directing errors to   the list maintainer rather than the message originator.   The text above implies that the final mail data will begin with a   return path line, followed by one or more time stamp lines.  These   lines will be followed by the rest of the mail data: first the   balance of the mail header section and then the body (RFC 5322 [4]).   It is sometimes difficult for an SMTP server to determine whether or   not it is making final delivery since forwarding or other operations   may occur after the message is accepted for delivery.  Consequently,   any further (forwarding, gateway, or relay) systems MAY remove the   return path and rebuild the MAIL command as needed to ensure that   exactly one such line appears in a delivered message.   A message-originating SMTP system SHOULD NOT send a message that   already contains a Return-path header field.  SMTP servers performing   a relay function MUST NOT inspect the message data, and especially   not to the extent needed to determine if Return-path header fields   are present.  SMTP servers making final delivery MAY remove Return-   path header fields before adding their own.   The primary purpose of the Return-path is to designate the address to   which messages indicating non-delivery or other mail system failures   are to be sent.  For this to be unambiguous, exactly one return path   SHOULD be present when the message is delivered.  Systems usingRFC822 syntax with non-SMTP transports SHOULD designate an unambiguous   address, associated with the transport envelope, to which error   reports (e.g., non-delivery messages) should be sent.Klensin                     Standards Track                    [Page 58]

RFC 5321                          SMTP                      October 2008   Historical note: Text inRFC 822 that appears to contradict the use   of the Return-path header field (or the envelope reverse-path address   from the MAIL command) as the destination for error messages is not   applicable on the Internet.  The reverse-path address (as copied into   the Return-path) MUST be used as the target of any mail containing   delivery error messages.   In particular:   o  a gateway from SMTP -> elsewhere SHOULD insert a return-path      header field, unless it is known that the "elsewhere" transport      also uses Internet domain addresses and maintains the envelope      sender address separately.   o  a gateway from elsewhere -> SMTP SHOULD delete any return-path      header field present in the message, and either copy that      information to the SMTP envelope or combine it with information      present in the envelope of the other transport system to construct      the reverse-path argument to the MAIL command in the SMTP      envelope.   The server must give special treatment to cases in which the   processing following the end of mail data indication is only   partially successful.  This could happen if, after accepting several   recipients and the mail data, the SMTP server finds that the mail   data could be successfully delivered to some, but not all, of the   recipients.  In such cases, the response to the DATA command MUST be   an OK reply.  However, the SMTP server MUST compose and send an   "undeliverable mail" notification message to the originator of the   message.   A single notification listing all of the failed recipients or   separate notification messages MUST be sent for each failed   recipient.  For economy of processing by the sender, the former   SHOULD be used when possible.  Note that the key difference between   handling aliases (Section 3.9.1) and forwarding (this subsection) is   the change to the backward-pointing address in this case.  All   notification messages about undeliverable mail MUST be sent using the   MAIL command (even if they result from processing the obsolete SEND,   SOML, or SAML commands) and MUST use a null return path as discussed   inSection 3.6.   The time stamp line and the return path line are formally defined as   follows (the definitions for "FWS" and "CFWS" appear inRFC 5322   [4]):   Return-path-line  = "Return-Path:" FWS Reverse-path <CRLF>   Time-stamp-line  = "Received:" FWS Stamp <CRLF>Klensin                     Standards Track                    [Page 59]

RFC 5321                          SMTP                      October 2008   Stamp          = From-domain By-domain Opt-info [CFWS] ";"                  FWS date-time                  ; where "date-time" is as defined inRFC 5322 [4]                  ; but the "obs-" forms, especially two-digit                  ; years, are prohibited in SMTP and MUST NOT be used.   From-domain    = "FROM" FWS Extended-Domain   By-domain      = CFWS "BY" FWS Extended-Domain   Extended-Domain  = Domain /                    ( Domain FWS "(" TCP-info ")" ) /                    ( address-literal FWS "(" TCP-info ")" )   TCP-info       = address-literal / ( Domain FWS address-literal )                  ; Information derived by server from TCP connection                  ; not client EHLO.   Opt-info       = [Via] [With] [ID] [For]                  [Additional-Registered-Clauses]   Via            = CFWS "VIA" FWS Link   With           = CFWS "WITH" FWS Protocol   ID             = CFWS "ID" FWS ( Atom / msg-id )                  ; msg-id is defined inRFC 5322 [4]   For            = CFWS "FOR" FWS ( Path / Mailbox )   Additional-Registered-Clauses  = CFWS Atom FWS String                                  ; Additional standard clauses may be                                  added in this                                  ; location by future standards and                                  registration with                                  ; IANA.  SMTP servers SHOULD NOT use                                  unregistered                                  ; names.  SeeSection 8.   Link           = "TCP" / Addtl-Link   Addtl-Link     = Atom                  ; Additional standard names for links are                  ; registered with the Internet Assigned Numbers                  ; Authority (IANA).  "Via" is primarily of value                  ; with non-Internet transports.  SMTP servers                  ; SHOULD NOT use unregistered names.Klensin                     Standards Track                    [Page 60]

RFC 5321                          SMTP                      October 2008   Protocol       = "ESMTP" / "SMTP" / Attdl-Protocol   Attdl-Protocol = Atom                  ; Additional standard names for protocols are                  ; registered with the Internet Assigned Numbers                  ; Authority (IANA) in the "mail parameters"                  ; registry [9].  SMTP servers SHOULD NOT                  ; use unregistered names.4.5.  Additional Implementation Issues4.5.1.  Minimum Implementation   In order to make SMTP workable, the following minimum implementation   MUST be provided by all receivers.  The following commands MUST be   supported to conform to this specification:      EHLO      HELO      MAIL      RCPT      DATA      RSET      NOOP      QUIT      VRFY   Any system that includes an SMTP server supporting mail relaying or   delivery MUST support the reserved mailbox "postmaster" as a case-   insensitive local name.  This postmaster address is not strictly   necessary if the server always returns 554 on connection opening (as   described inSection 3.1).  The requirement to accept mail for   postmaster implies that RCPT commands that specify a mailbox for   postmaster at any of the domains for which the SMTP server provides   mail service, as well as the special case of "RCPT TO:<Postmaster>"   (with no domain specification), MUST be supported.   SMTP systems are expected to make every reasonable effort to accept   mail directed to Postmaster from any other system on the Internet.   In extreme cases -- such as to contain a denial of service attack or   other breach of security -- an SMTP server may block mail directed to   Postmaster.  However, such arrangements SHOULD be narrowly tailored   so as to avoid blocking messages that are not part of such attacks.Klensin                     Standards Track                    [Page 61]

RFC 5321                          SMTP                      October 20084.5.2.  Transparency   Without some provision for data transparency, the character sequence   "<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.   In general, users are not aware of such "forbidden" sequences.  To   allow all user composed text to be transmitted transparently, the   following procedures are used:   o  Before sending a line of mail text, the SMTP client checks the      first character of the line.  If it is a period, one additional      period is inserted at the beginning of the line.   o  When a line of mail text is received by the SMTP server, it checks      the line.  If the line is composed of a single period, it is      treated as the end of mail indicator.  If the first character is a      period and there are other characters on the line, the first      character is deleted.   The mail data may contain any of the 128 ASCII characters.  All   characters are to be delivered to the recipient's mailbox, including   spaces, vertical and horizontal tabs, and other control characters.   If the transmission channel provides an 8-bit byte (octet) data   stream, the 7-bit ASCII codes are transmitted, right justified, in   the octets, with the high-order bits cleared to zero.  SeeSection 3.6 for special treatment of these conditions in SMTP systems   serving a relay function.   In some systems, it may be necessary to transform the data as it is   received and stored.  This may be necessary for hosts that use a   different character set than ASCII as their local character set, that   store data in records rather than strings, or which use special   character sequences as delimiters inside mailboxes.  If such   transformations are necessary, they MUST be reversible, especially if   they are applied to mail being relayed.4.5.3.  Sizes and Timeouts4.5.3.1.  Size Limits and Minimums   There are several objects that have required minimum/maximum sizes.   Every implementation MUST be able to receive objects of at least   these sizes.  Objects larger than these sizes SHOULD be avoided when   possible.  However, some Internet mail constructs such as encoded   X.400 addresses (RFC 2156 [35]) will often require larger objects.   Clients MAY attempt to transmit these, but MUST be prepared for a   server to reject them if they cannot be handled by it.  To the   maximum extent possible, implementation techniques that impose no   limits on the length of these objects should be used.Klensin                     Standards Track                    [Page 62]

RFC 5321                          SMTP                      October 2008   Extensions to SMTP may involve the use of characters that occupy more   than a single octet each.  This section therefore specifies lengths   in octets where absolute lengths, rather than character counts, are   intended.4.5.3.1.1.  Local-part   The maximum total length of a user name or other local-part is 64   octets.4.5.3.1.2.  Domain   The maximum total length of a domain name or number is 255 octets.4.5.3.1.3.  Path   The maximum total length of a reverse-path or forward-path is 256   octets (including the punctuation and element separators).4.5.3.1.4.  Command Line   The maximum total length of a command line including the command word   and the <CRLF> is 512 octets.  SMTP extensions may be used to   increase this limit.4.5.3.1.5.  Reply Line   The maximum total length of a reply line including the reply code and   the <CRLF> is 512 octets.  More information may be conveyed through   multiple-line replies.4.5.3.1.6.  Text Line   The maximum total length of a text line including the <CRLF> is 1000   octets (not counting the leading dot duplicated for transparency).   This number may be increased by the use of SMTP Service Extensions.4.5.3.1.7.  Message Content   The maximum total length of a message content (including any message   header section as well as the message body) MUST BE at least 64K   octets.  Since the introduction of Internet Standards for multimedia   mail (RFC 2045 [21]), message lengths on the Internet have grown   dramatically, and message size restrictions should be avoided if at   all possible.  SMTP server systems that must impose restrictions   SHOULD implement the "SIZE" service extension ofRFC 1870 [10], and   SMTP client systems that will send large messages SHOULD utilize it   when possible.Klensin                     Standards Track                    [Page 63]

RFC 5321                          SMTP                      October 20084.5.3.1.8.  Recipients Buffer   The minimum total number of recipients that MUST be buffered is 100   recipients.  Rejection of messages (for excessive recipients) with   fewer than 100 RCPT commands is a violation of this specification.   The general principle that relaying SMTP server MUST NOT, and   delivery SMTP servers SHOULD NOT, perform validation tests on message   header fields suggests that messages SHOULD NOT be rejected based on   the total number of recipients shown in header fields.  A server that   imposes a limit on the number of recipients MUST behave in an orderly   fashion, such as rejecting additional addresses over its limit rather   than silently discarding addresses previously accepted.  A client   that needs to deliver a message containing over 100 RCPT commands   SHOULD be prepared to transmit in 100-recipient "chunks" if the   server declines to accept more than 100 recipients in a single   message.4.5.3.1.9.  Treatment When Limits Exceeded   Errors due to exceeding these limits may be reported by using the   reply codes.  Some examples of reply codes are:      500 Line too long.   or      501 Path too long   or      452 Too many recipients (see below)   or      552 Too much mail data.4.5.3.1.10.  Too Many Recipients CodeRFC 821 [1] incorrectly listed the error where an SMTP server   exhausts its implementation limit on the number of RCPT commands   ("too many recipients") as having reply code 552.  The correct reply   code for this condition is 452.  Clients SHOULD treat a 552 code in   this case as a temporary, rather than permanent, failure so the logic   below works.   When a conforming SMTP server encounters this condition, it has at   least 100 successful RCPT commands in its recipients buffer.  If the   server is able to accept the message, then at least these 100Klensin                     Standards Track                    [Page 64]

RFC 5321                          SMTP                      October 2008   addresses will be removed from the SMTP client's queue.  When the   client attempts retransmission of those addresses that received 452   responses, at least 100 of these will be able to fit in the SMTP   server's recipients buffer.  Each retransmission attempt that is able   to deliver anything will be able to dispose of at least 100 of these   recipients.   If an SMTP server has an implementation limit on the number of RCPT   commands and this limit is exhausted, it MUST use a response code of   452 (but the client SHOULD also be prepared for a 552, as noted   above).  If the server has a configured site-policy limitation on the   number of RCPT commands, it MAY instead use a 5yz response code.  In   particular, if the intent is to prohibit messages with more than a   site-specified number of recipients, rather than merely limit the   number of recipients in a given mail transaction, it would be   reasonable to return a 503 response to any DATA command received   subsequent to the 452 (or 552) code or to simply return the 503 after   DATA without returning any previous negative response.4.5.3.2.  Timeouts   An SMTP client MUST provide a timeout mechanism.  It MUST use per-   command timeouts rather than somehow trying to time the entire mail   transaction.  Timeouts SHOULD be easily reconfigurable, preferably   without recompiling the SMTP code.  To implement this, a timer is set   for each SMTP command and for each buffer of the data transfer.  The   latter means that the overall timeout is inherently proportional to   the size of the message.   Based on extensive experience with busy mail-relay hosts, the minimum   per-command timeout values SHOULD be as follows:4.5.3.2.1.  Initial 220 Message: 5 Minutes   An SMTP client process needs to distinguish between a failed TCP   connection and a delay in receiving the initial 220 greeting message.   Many SMTP servers accept a TCP connection but delay delivery of the   220 message until their system load permits more mail to be   processed.4.5.3.2.2.  MAIL Command: 5 Minutes4.5.3.2.3.  RCPT Command: 5 Minutes   A longer timeout is required if processing of mailing lists and   aliases is not deferred until after the message was accepted.Klensin                     Standards Track                    [Page 65]

RFC 5321                          SMTP                      October 20084.5.3.2.4.  DATA Initiation: 2 Minutes   This is while awaiting the "354 Start Input" reply to a DATA command.4.5.3.2.5.  Data Block: 3 Minutes   This is while awaiting the completion of each TCP SEND call   transmitting a chunk of data.4.5.3.2.6.  DATA Termination: 10 Minutes.   This is while awaiting the "250 OK" reply.  When the receiver gets   the final period terminating the message data, it typically performs   processing to deliver the message to a user mailbox.  A spurious   timeout at this point would be very wasteful and would typically   result in delivery of multiple copies of the message, since it has   been successfully sent and the server has accepted responsibility for   delivery.  SeeSection 6.1 for additional discussion.4.5.3.2.7.  Server Timeout: 5 Minutes.   An SMTP server SHOULD have a timeout of at least 5 minutes while it   is awaiting the next command from the sender.4.5.4.  Retry Strategies   The common structure of a host SMTP implementation includes user   mailboxes, one or more areas for queuing messages in transit, and one   or more daemon processes for sending and receiving mail.  The exact   structure will vary depending on the needs of the users on the host   and the number and size of mailing lists supported by the host.  We   describe several optimizations that have proved helpful, particularly   for mailers supporting high traffic levels.   Any queuing strategy MUST include timeouts on all activities on a   per-command basis.  A queuing strategy MUST NOT send error messages   in response to error messages under any circumstances.4.5.4.1.  Sending Strategy   The general model for an SMTP client is one or more processes that   periodically attempt to transmit outgoing mail.  In a typical system,   the program that composes a message has some method for requesting   immediate attention for a new piece of outgoing mail, while mail that   cannot be transmitted immediately MUST be queued and periodically   retried by the sender.  A mail queue entry will include not only the   message itself but also the envelope information.Klensin                     Standards Track                    [Page 66]

RFC 5321                          SMTP                      October 2008   The sender MUST delay retrying a particular destination after one   attempt has failed.  In general, the retry interval SHOULD be at   least 30 minutes; however, more sophisticated and variable strategies   will be beneficial when the SMTP client can determine the reason for   non-delivery.   Retries continue until the message is transmitted or the sender gives   up; the give-up time generally needs to be at least 4-5 days.  It MAY   be appropriate to set a shorter maximum number of retries for non-   delivery notifications and equivalent error messages than for   standard messages.  The parameters to the retry algorithm MUST be   configurable.   A client SHOULD keep a list of hosts it cannot reach and   corresponding connection timeouts, rather than just retrying queued   mail items.   Experience suggests that failures are typically transient (the target   system or its connection has crashed), favoring a policy of two   connection attempts in the first hour the message is in the queue,   and then backing off to one every two or three hours.   The SMTP client can shorten the queuing delay in cooperation with the   SMTP server.  For example, if mail is received from a particular   address, it is likely that mail queued for that host can now be sent.   Application of this principle may, in many cases, eliminate the   requirement for an explicit "send queues now" function such as ETRN,RFC 1985 [36].   The strategy may be further modified as a result of multiple   addresses per host (see below) to optimize delivery time versus   resource usage.   An SMTP client may have a large queue of messages for each   unavailable destination host.  If all of these messages were retried   in every retry cycle, there would be excessive Internet overhead and   the sending system would be blocked for a long period.  Note that an   SMTP client can generally determine that a delivery attempt has   failed only after a timeout of several minutes, and even a one-minute   timeout per connection will result in a very large delay if retries   are repeated for dozens, or even hundreds, of queued messages to the   same host.   At the same time, SMTP clients SHOULD use great care in caching   negative responses from servers.  In an extreme case, if EHLO is   issued multiple times during the same SMTP connection, different   answers may be returned by the server.  More significantly, 5yz   responses to the MAIL command MUST NOT be cached.Klensin                     Standards Track                    [Page 67]

RFC 5321                          SMTP                      October 2008   When a mail message is to be delivered to multiple recipients, and   the SMTP server to which a copy of the message is to be sent is the   same for multiple recipients, then only one copy of the message   SHOULD be transmitted.  That is, the SMTP client SHOULD use the   command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the   sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA.  However, if there   are very many addresses, a limit on the number of RCPT commands per   MAIL command MAY be imposed.  This efficiency feature SHOULD be   implemented.   Similarly, to achieve timely delivery, the SMTP client MAY support   multiple concurrent outgoing mail transactions.  However, some limit   may be appropriate to protect the host from devoting all its   resources to mail.4.5.4.2.  Receiving Strategy   The SMTP server SHOULD attempt to keep a pending listen on the SMTP   port (specified by IANA as port 25) at all times.  This requires the   support of multiple incoming TCP connections for SMTP.  Some limit   MAY be imposed, but servers that cannot handle more than one SMTP   transaction at a time are not in conformance with the intent of this   specification.   As discussed above, when the SMTP server receives mail from a   particular host address, it could activate its own SMTP queuing   mechanisms to retry any mail pending for that host address.4.5.5.  Messages with a Null Reverse-Path   There are several types of notification messages that are required by   existing and proposed Standards to be sent with a null reverse-path,   namely non-delivery notifications as discussed inSection 3.7, other   kinds of Delivery Status Notifications (DSNs,RFC 3461 [32]), and   Message Disposition Notifications (MDNs,RFC 3798 [37]).  All of   these kinds of messages are notifications about a previous message,   and they are sent to the reverse-path of the previous mail message.   (If the delivery of such a notification message fails, that usually   indicates a problem with the mail system of the host to which the   notification message is addressed.  For this reason, at some hosts   the MTA is set up to forward such failed notification messages to   someone who is able to fix problems with the mail system, e.g., via   the postmaster alias.)   All other types of messages (i.e., any message which is not required   by a Standards-Track RFC to have a null reverse-path) SHOULD be sent   with a valid, non-null reverse-path.Klensin                     Standards Track                    [Page 68]

RFC 5321                          SMTP                      October 2008   Implementers of automated email processors should be careful to make   sure that the various kinds of messages with a null reverse-path are   handled correctly.  In particular, such systems SHOULD NOT reply to   messages with a null reverse-path, and they SHOULD NOT add a non-null   reverse-path, or change a null reverse-path to a non-null one, to   such messages when forwarding.5.  Address Resolution and Mail Handling5.1.  Locating the Target Host   Once an SMTP client lexically identifies a domain to which mail will   be delivered for processing (as described in Sections2.3.5 and3.6),   a DNS lookup MUST be performed to resolve the domain name (RFC 1035   [2]).  The names are expected to be fully-qualified domain names   (FQDNs): mechanisms for inferring FQDNs from partial names or local   aliases are outside of this specification.  Due to a history of   problems, SMTP servers used for initial submission of messages SHOULD   NOT make such inferences (Message Submission Servers [18] have   somewhat more flexibility) and intermediate (relay) SMTP servers MUST   NOT make them.   The lookup first attempts to locate an MX record associated with the   name.  If a CNAME record is found, the resulting name is processed as   if it were the initial name.  If a non-existent domain error is   returned, this situation MUST be reported as an error.  If a   temporary error is returned, the message MUST be queued and retried   later (seeSection 4.5.4.1).  If an empty list of MXs is returned,   the address is treated as if it was associated with an implicit MX   RR, with a preference of 0, pointing to that host.  If MX records are   present, but none of them are usable, or the implicit MX is unusable,   this situation MUST be reported as an error.   If one or more MX RRs are found for a given name, SMTP systems MUST   NOT utilize any address RRs associated with that name unless they are   located using the MX RRs; the "implicit MX" rule above applies only   if there are no MX records present.  If MX records are present, but   none of them are usable, this situation MUST be reported as an error.   When a domain name associated with an MX RR is looked up and the   associated data field obtained, the data field of that response MUST   contain a domain name.  That domain name, when queried, MUST return   at least one address record (e.g., A or AAAA RR) that gives the IP   address of the SMTP server to which the message should be directed.   Any other response, specifically including a value that will return a   CNAME record when queried, lies outside the scope of this Standard.   The prohibition on labels in the data that resolve to CNAMEs is   discussed in more detail inRFC 2181, Section 10.3 [38].Klensin                     Standards Track                    [Page 69]

RFC 5321                          SMTP                      October 2008   When the lookup succeeds, the mapping can result in a list of   alternative delivery addresses rather than a single address, because   of multiple MX records, multihoming, or both.  To provide reliable   mail transmission, the SMTP client MUST be able to try (and retry)   each of the relevant addresses in this list in order, until a   delivery attempt succeeds.  However, there MAY also be a configurable   limit on the number of alternate addresses that can be tried.  In any   case, the SMTP client SHOULD try at least two addresses.   Two types of information are used to rank the host addresses:   multiple MX records, and multihomed hosts.   MX records contain a preference indication that MUST be used in   sorting if more than one such record appears (see below).  Lower   numbers are more preferred than higher ones.  If there are multiple   destinations with the same preference and there is no clear reason to   favor one (e.g., by recognition of an easily reached address), then   the sender-SMTP MUST randomize them to spread the load across   multiple mail exchangers for a specific organization.   The destination host (perhaps taken from the preferred MX record) may   be multihomed, in which case the domain name resolver will return a   list of alternative IP addresses.  It is the responsibility of the   domain name resolver interface to have ordered this list by   decreasing preference if necessary, and the SMTP sender MUST try them   in the order presented.   Although the capability to try multiple alternative addresses is   required, specific installations may want to limit or disable the use   of alternative addresses.  The question of whether a sender should   attempt retries using the different addresses of a multihomed host   has been controversial.  The main argument for using the multiple   addresses is that it maximizes the probability of timely delivery,   and indeed sometimes the probability of any delivery; the counter-   argument is that it may result in unnecessary resource use.  Note   that resource use is also strongly determined by the sending strategy   discussed inSection 4.5.4.1.   If an SMTP server receives a message with a destination for which it   is a designated Mail eXchanger, it MAY relay the message (potentially   after having rewritten the MAIL FROM and/or RCPT TO addresses), make   final delivery of the message, or hand it off using some mechanism   outside the SMTP-provided transport environment.  Of course, neither   of the latter require that the list of MX records be examined   further.   If it determines that it should relay the message without rewriting   the address, it MUST sort the MX records to determine candidates forKlensin                     Standards Track                    [Page 70]

RFC 5321                          SMTP                      October 2008   delivery.  The records are first ordered by preference, with the   lowest-numbered records being most preferred.  The relay host MUST   then inspect the list for any of the names or addresses by which it   might be known in mail transactions.  If a matching record is found,   all records at that preference level and higher-numbered ones MUST be   discarded from consideration.  If there are no records left at that   point, it is an error condition, and the message MUST be returned as   undeliverable.  If records do remain, they SHOULD be tried, best   preference first, as described above.5.2.  IPv6 and MX Records   In the contemporary Internet, SMTP clients and servers may be hosted   on IPv4 systems, IPv6 systems, or dual-stack systems that are   compatible with either version of the Internet Protocol.  The host   domains to which MX records point may, consequently, contain "A RR"s   (IPv4), "AAAA RR"s (IPv6), or any combination of them.  WhileRFC3974 [39] discusses some operational experience in mixed   environments, it was not comprehensive enough to justify   standardization, and some of its recommendations appear to be   inconsistent with this specification.  The appropriate actions to be   taken either will depend on local circumstances, such as performance   of the relevant networks and any conversions that might be necessary,   or will be obvious (e.g., an IPv6-only client need not attempt to   look up A RRs or attempt to reach IPv4-only servers).  Designers of   SMTP implementations that might run in IPv6 or dual-stack   environments should study the procedures above, especially the   comments about multihomed hosts, and, preferably, provide mechanisms   to facilitate operational tuning and mail interoperability between   IPv4 and IPv6 systems while considering local circumstances.6.  Problem Detection and Handling6.1.  Reliable Delivery and Replies by Email   When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"   message in response to DATA), it is accepting responsibility for   delivering or relaying the message.  It must take this responsibility   seriously.  It MUST NOT lose the message for frivolous reasons, such   as because the host later crashes or because of a predictable   resource shortage.  Some reasons that are not considered frivolous   are discussed in the next subsection and inSection 7.8.   If there is a delivery failure after acceptance of a message, the   receiver-SMTP MUST formulate and mail a notification message.  This   notification MUST be sent using a null ("<>") reverse-path in the   envelope.  The recipient of this notification MUST be the address   from the envelope return path (or the Return-Path: line).  However,Klensin                     Standards Track                    [Page 71]

RFC 5321                          SMTP                      October 2008   if this address is null ("<>"), the receiver-SMTP MUST NOT send a   notification.  Obviously, nothing in this section can or should   prohibit local decisions (i.e., as part of the same system   environment as the receiver-SMTP) to log or otherwise transmit   information about null address events locally if that is desired.  If   the address is an explicit source route, it MUST be stripped down to   its final hop.   For example, suppose that an error notification must be sent for a   message that arrived with:      MAIL FROM:<@a,@b:user@d>   The notification message MUST be sent using:      RCPT TO:<user@d>   Some delivery failures after the message is accepted by SMTP will be   unavoidable.  For example, it may be impossible for the receiving   SMTP server to validate all the delivery addresses in RCPT command(s)   due to a "soft" domain system error, because the target is a mailing   list (see earlier discussion of RCPT), or because the server is   acting as a relay and has no immediate access to the delivering   system.   To avoid receiving duplicate messages as the result of timeouts, a   receiver-SMTP MUST seek to minimize the time required to respond to   the final <CRLF>.<CRLF> end of data indicator.  SeeRFC 1047 [40] for   a discussion of this problem.6.2.  Unwanted, Unsolicited, and "Attack" Messages   Utility and predictability of the Internet mail system requires that   messages that can be delivered should be delivered, regardless of any   syntax or other faults associated with those messages and regardless   of their content.  If they cannot be delivered, and cannot be   rejected by the SMTP server during the SMTP transaction, they should   be "bounced" (returned with non-delivery notification messages) as   described above.  In today's world, in which many SMTP server   operators have discovered that the quantity of undesirable bulk email   vastly exceeds the quantity of desired mail and in which accepting a   message may trigger additional undesirable traffic by providing   verification of the address, those principles may not be practical.   As discussed inSection 7.8 andSection 7.9 below, dropping mail   without notification of the sender is permitted in practice.   However, it is extremely dangerous and violates a long tradition and   community expectations that mail is either delivered or returned.  IfKlensin                     Standards Track                    [Page 72]

RFC 5321                          SMTP                      October 2008   silent message-dropping is misused, it could easily undermine   confidence in the reliability of the Internet's mail systems.  So   silent dropping of messages should be considered only in those cases   where there is very high confidence that the messages are seriously   fraudulent or otherwise inappropriate.   To stretch the principle of delivery if possible even further, it may   be a rational policy to not deliver mail that has an invalid return   address, although the history of the network is that users are   typically better served by delivering any message that can be   delivered.  Reliably determining that a return address is invalid can   be a difficult and time-consuming process, especially if the putative   sending system is not directly accessible or does not fully and   accurately support VRFY and, even if a "drop messages with invalid   return addresses" policy is adopted, it SHOULD be applied only when   there is near-certainty that the return addresses are, in fact,   invalid.   Conversely, if a message is rejected because it is found to contain   hostile content (a decision that is outside the scope of an SMTP   server as defined in this document), rejection ("bounce") messages   SHOULD NOT be sent unless the receiving site is confident that those   messages will be usefully delivered.  The preference and default in   these cases is to avoid sending non-delivery messages when the   incoming message is determined to contain hostile content.6.3.  Loop Detection   Simple counting of the number of "Received:" header fields in a   message has proven to be an effective, although rarely optimal,   method of detecting loops in mail systems.  SMTP servers using this   technique SHOULD use a large rejection threshold, normally at least   100 Received entries.  Whatever mechanisms are used, servers MUST   contain provisions for detecting and stopping trivial loops.6.4.  Compensating for Irregularities   Unfortunately, variations, creative interpretations, and outright   violations of Internet mail protocols do occur; some would suggest   that they occur quite frequently.  The debate as to whether a well-   behaved SMTP receiver or relay should reject a malformed message,   attempt to pass it on unchanged, or attempt to repair it to increase   the odds of successful delivery (or subsequent reply) began almost   with the dawn of structured network mail and shows no signs of   abating.  Advocates of rejection claim that attempted repairs are   rarely completely adequate and that rejection of bad messages is the   only way to get the offending software repaired.  Advocates of   "repair" or "deliver no matter what" argue that users prefer thatKlensin                     Standards Track                    [Page 73]

RFC 5321                          SMTP                      October 2008   mail go through it if at all possible and that there are significant   market pressures in that direction.  In practice, these market   pressures may be more important to particular vendors than strict   conformance to the standards, regardless of the preference of the   actual developers.   The problems associated with ill-formed messages were exacerbated by   the introduction of the split-UA mail reading protocols (Post Office   Protocol (POP) version 2 [15], Post Office Protocol (POP) version 3   [16], IMAP version 2 [41], and PCMAIL [42]).  These protocols   encouraged the use of SMTP as a posting (message submission)   protocol, and SMTP servers as relay systems for these client hosts   (which are often only intermittently connected to the Internet).   Historically, many of those client machines lacked some of the   mechanisms and information assumed by SMTP (and indeed, by the mail   format protocol,RFC 822 [28]).  Some could not keep adequate track   of time; others had no concept of time zones; still others could not   identify their own names or addresses; and, of course, none could   satisfy the assumptions that underlayRFC 822's conception of   authenticated addresses.   In response to these weak SMTP clients, many SMTP systems now   complete messages that are delivered to them in incomplete or   incorrect form.  This strategy is generally considered appropriate   when the server can identify or authenticate the client, and there   are prior agreements between them.  By contrast, there is at best   great concern about fixes applied by a relay or delivery SMTP server   that has little or no knowledge of the user or client machine.  Many   of these issues are addressed by using a separate protocol, such as   that defined inRFC 4409 [18], for message submission, rather than   using originating SMTP servers for that purpose.   The following changes to a message being processed MAY be applied   when necessary by an originating SMTP server, or one used as the   target of SMTP as an initial posting (message submission) protocol:   o  Addition of a message-id field when none appears   o  Addition of a date, time, or time zone when none appears   o  Correction of addresses to proper FQDN format   The less information the server has about the client, the less likely   these changes are to be correct and the more caution and conservatism   should be applied when considering whether or not to perform fixes   and how.  These changes MUST NOT be applied by an SMTP server that   provides an intermediate relay function.Klensin                     Standards Track                    [Page 74]

RFC 5321                          SMTP                      October 2008   In all cases, properly operating clients supplying correct   information are preferred to corrections by the SMTP server.  In all   cases, documentation SHOULD be provided in trace header fields and/or   header field comments for actions performed by the servers.7.  Security Considerations7.1.  Mail Security and Spoofing   SMTP mail is inherently insecure in that it is feasible for even   fairly casual users to negotiate directly with receiving and relaying   SMTP servers and create messages that will trick a naive recipient   into believing that they came from somewhere else.  Constructing such   a message so that the "spoofed" behavior cannot be detected by an   expert is somewhat more difficult, but not sufficiently so as to be a   deterrent to someone who is determined and knowledgeable.   Consequently, as knowledge of Internet mail increases, so does the   knowledge that SMTP mail inherently cannot be authenticated, or   integrity checks provided, at the transport level.  Real mail   security lies only in end-to-end methods involving the message   bodies, such as those that use digital signatures (seeRFC 1847 [43]   and, e.g., Pretty Good Privacy (PGP) inRFC 4880 [44] or Secure/   Multipurpose Internet Mail Extensions (S/MIME) inRFC 3851 [45]).   Various protocol extensions and configuration options that provide   authentication at the transport level (e.g., from an SMTP client to   an SMTP server) improve somewhat on the traditional situation   described above.  However, in general, they only authenticate one   server to another rather than a chain of relays and servers, much   less authenticating users or user machines.  Consequently, unless   they are accompanied by careful handoffs of responsibility in a   carefully designed trust environment, they remain inherently weaker   than end-to-end mechanisms that use digitally signed messages rather   than depending on the integrity of the transport system.   Efforts to make it more difficult for users to set envelope return   path and header "From" fields to point to valid addresses other than   their own are largely misguided: they frustrate legitimate   applications in which mail is sent by one user on behalf of another,   in which error (or normal) replies should be directed to a special   address, or in which a single message is sent to multiple recipients   on different hosts.  (Systems that provide convenient ways for users   to alter these header fields on a per-message basis should attempt to   establish a primary and permanent mailbox address for the user so   that Sender header fields within the message data can be generated   sensibly.)Klensin                     Standards Track                    [Page 75]

RFC 5321                          SMTP                      October 2008   This specification does not further address the authentication issues   associated with SMTP other than to advocate that useful functionality   not be disabled in the hope of providing some small margin of   protection against a user who is trying to fake mail.7.2.  "Blind" Copies   Addresses that do not appear in the message header section may appear   in the RCPT commands to an SMTP server for a number of reasons.  The   two most common involve the use of a mailing address as a "list   exploder" (a single address that resolves into multiple addresses)   and the appearance of "blind copies".  Especially when more than one   RCPT command is present, and in order to avoid defeating some of the   purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy   the full set of RCPT command arguments into the header section,   either as part of trace header fields or as informational or private-   extension header fields.  Since this rule is often violated in   practice, and cannot be enforced, sending SMTP systems that are aware   of "bcc" use MAY find it helpful to send each blind copy as a   separate message transaction containing only a single RCPT command.   There is no inherent relationship between either "reverse" (from   MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP   transaction ("envelope") and the addresses in the header section.   Receiving systems SHOULD NOT attempt to deduce such relationships and   use them to alter the header section of the message for delivery.   The popular "Apparently-to" header field is a violation of this   principle as well as a common source of unintended information   disclosure and SHOULD NOT be used.7.3.  VRFY, EXPN, and Security   As discussed inSection 3.5, individual sites may want to disable   either or both of VRFY or EXPN for security reasons (see below).  As   a corollary to the above, implementations that permit this MUST NOT   appear to have verified addresses that are not, in fact, verified.   If a site disables these commands for security reasons, the SMTP   server MUST return a 252 response, rather than a code that could be   confused with successful or unsuccessful verification.   Returning a 250 reply code with the address listed in the VRFY   command after having checked it only for syntax violates this rule.   Of course, an implementation that "supports" VRFY by always returning   550 whether or not the address is valid is equally not in   conformance.   On the public Internet, the contents of mailing lists have become   popular as an address information source for so-called "spammers."Klensin                     Standards Track                    [Page 76]

RFC 5321                          SMTP                      October 2008   The use of EXPN to "harvest" addresses has increased as list   administrators have installed protections against inappropriate uses   of the lists themselves.  However, VRFY and EXPN are still useful for   authenticated users and within an administrative domain.  For   example, VRFY and EXPN are useful for performing internal audits of   how email gets routed to check and to make sure no one is   automatically forwarding sensitive mail outside the organization.   Sites implementing SMTP authentication may choose to make VRFY and   EXPN available only to authenticated requestors.  Implementations   SHOULD still provide support for EXPN, but sites SHOULD carefully   evaluate the tradeoffs.   Whether disabling VRFY provides any real marginal security depends on   a series of other conditions.  In many cases, RCPT commands can be   used to obtain the same information about address validity.  On the   other hand, especially in situations where determination of address   validity for RCPT commands is deferred until after the DATA command   is received, RCPT may return no information at all, while VRFY is   expected to make a serious attempt to determine validity before   generating a response code (see discussion above).7.4.  Mail Rerouting Based on the 251 and 551 Response Codes   Before a client uses the 251 or 551 reply codes from a RCPT command   to automatically update its future behavior (e.g., updating the   user's address book), it should be certain of the server's   authenticity.  If it does not, it may be subject to a man in the   middle attack.7.5.  Information Disclosure in Announcements   There has been an ongoing debate about the tradeoffs between the   debugging advantages of announcing server type and version (and,   sometimes, even server domain name) in the greeting response or in   response to the HELP command and the disadvantages of exposing   information that might be useful in a potential hostile attack.  The   utility of the debugging information is beyond doubt.  Those who   argue for making it available point out that it is far better to   actually secure an SMTP server rather than hope that trying to   conceal known vulnerabilities by hiding the server's precise identity   will provide more protection.  Sites are encouraged to evaluate the   tradeoff with that issue in mind; implementations SHOULD minimally   provide for making type and version information available in some way   to other network hosts.Klensin                     Standards Track                    [Page 77]

RFC 5321                          SMTP                      October 20087.6.  Information Disclosure in Trace Fields   In some circumstances, such as when mail originates from within a LAN   whose hosts are not directly on the public Internet, trace   ("Received") header fields produced in conformance with this   specification may disclose host names and similar information that   would not normally be available.  This ordinarily does not pose a   problem, but sites with special concerns about name disclosure should   be aware of it.  Also, the optional FOR clause should be supplied   with caution or not at all when multiple recipients are involved lest   it inadvertently disclose the identities of "blind copy" recipients   to others.7.7.  Information Disclosure in Message Forwarding   As discussed inSection 3.4, use of the 251 or 551 reply codes to   identify the replacement address associated with a mailbox may   inadvertently disclose sensitive information.  Sites that are   concerned about those issues should ensure that they select and   configure servers appropriately.7.8.  Resistance to Attacks   In recent years, there has been an increase of attacks on SMTP   servers, either in conjunction with attempts to discover addresses   for sending unsolicited messages or simply to make the servers   inaccessible to others (i.e., as an application-level denial of   service attack).  While the means of doing so are beyond the scope of   this Standard, rational operational behavior requires that servers be   permitted to detect such attacks and take action to defend   themselves.  For example, if a server determines that a large number   of RCPT TO commands are being sent, most or all with invalid   addresses, as part of such an attack, it would be reasonable for the   server to close the connection after generating an appropriate number   of 5yz (normally 550) replies.7.9.  Scope of Operation of SMTP Servers   It is a well-established principle that an SMTP server may refuse to   accept mail for any operational or technical reason that makes sense   to the site providing the server.  However, cooperation among sites   and installations makes the Internet possible.  If sites take   excessive advantage of the right to reject traffic, the ubiquity of   email availability (one of the strengths of the Internet) will be   threatened; considerable care should be taken and balance maintained   if a site decides to be selective about the traffic it will accept   and process.Klensin                     Standards Track                    [Page 78]

RFC 5321                          SMTP                      October 2008   In recent years, use of the relay function through arbitrary sites   has been used as part of hostile efforts to hide the actual origins   of mail.  Some sites have decided to limit the use of the relay   function to known or identifiable sources, and implementations SHOULD   provide the capability to perform this type of filtering.  When mail   is rejected for these or other policy reasons, a 550 code SHOULD be   used in response to EHLO (or HELO), MAIL, or RCPT as appropriate.8.  IANA Considerations   IANA maintains three registries in support of this specification, all   of which were created forRFC 2821 or earlier.  This document expands   the third one as specified below.  The registry references listed are   as of the time of publication; IANA does not guarantee the locations   associated with the URLs.  The registries are as follows:   o  The first, "Simple Mail Transfer Protocol (SMTP) Service      Extensions" [46], consists of SMTP service extensions with the      associated keywords, and, as needed, parameters and verbs.  As      specified inSection 2.2.2, no entry may be made in this registry      that starts in an "X".  Entries may be made only for service      extensions (and associated keywords, parameters, or verbs) that      are defined in Standards-Track or Experimental RFCs specifically      approved by the IESG for this purpose.   o  The second registry, "Address Literal Tags" [47], consists of      "tags" that identify forms of domain literals other than those for      IPv4 addresses (specified inRFC 821 and in this document).  The      initial entry in that registry is for IPv6 addresses (specified in      this document).  Additional literal types require standardization      before being used; none are anticipated at this time.   o  The third, "Mail Transmission Types" [46], established byRFC 821      and renewed by this specification, is a registry of link and      protocol identifiers to be used with the "via" and "with"      subclauses of the time stamp ("Received:" header field) described      inSection 4.4.  Link and protocol identifiers in addition to      those specified in this document may be registered only by      standardization or by way of an RFC-documented, IESG-approved,      Experimental protocol extension.  This name space is for      identification and not limited in size: the IESG is encouraged to      approve on the basis of clear documentation and a distinct method      rather than preferences about the properties of the method itself.      An additional subsection has been added to the "VIA link types"      and "WITH protocol types" subsections of this registry to contain      registrations of "Additional-registered-clauses" as described      above.  The registry will contain clause names, a description, aKlensin                     Standards Track                    [Page 79]

RFC 5321                          SMTP                      October 2008      summary of the syntax of the associated String, and a reference.      As new clauses are defined, they may, in principle, specify      creation of their own registries if the Strings consist of      reserved terms or keywords rather than less restricted strings.      As with link and protocol identifiers, additional clauses may be      registered only by standardization or by way of an RFC-documented,      IESG-approved, Experimental protocol extension.  The additional      clause name space is for identification and is not limited in      size: the IESG is encouraged to approve on the basis of clear      documentation, actual use or strong signs that the clause will be      used, and a distinct requirement rather than preferences about the      properties of the clause itself.   In addition, if additional trace header fields (i.e., in addition to   Return-path and Received) are ever created, those trace fields MUST   be added to the IANA registry established byBCP 90 (RFC 3864) [11]   for use withRFC 5322 [4].9.  Acknowledgments   Many people contributed to the development ofRFC 2821.  That   document should be consulted for those acknowledgments.  For the   present document, the editor and the community owe thanks to Dawn   Mann and Tony Hansen who assisted in the very painful process of   editing and converting the internal format of the document from one   system to another.   Neither this document norRFC 2821 would have been possible without   the many contribution and insights of the late Jon Postel.  Those   contributions of course include the original specification of SMTP inRFC 821.  A considerable quantity of text fromRFC 821 still appears   in this document as do several of Jon's original examples that have   been updated only as needed to reflect other changes in the   specification.   Many people made comments or suggestions on the mailing list or in   notes to the author.  Important corrections or clarifications were   suggested by several people, including Matti Aarnio, Glenn Anderson,   Derek J. Balling, Alex van den Bogaerdt, Stephane Bortzmeyer, Vint   Cerf, Jutta Degener, Steve Dorner, Lisa Dusseault, Frank Ellerman,   Ned Freed, Randy Gellens, Sabahattin Gucukoglu, Philip Guenther, Arnt   Gulbrandsen, Eric Hall, Richard O. Hammer, Tony Hansen, Peter J.   Holzer, Kari Hurtta, Bryon Roche Kain, Valdis Kletnieks, Mathias   Koerber, John Leslie, Bruce Lilly, Jeff Macdonald, Mark E. Mallett,   Mark Martinec, S. Moonesamy, Lyndon Nerenberg, Chris Newman, Douglas   Otis, Pete Resnick, Robert A. Rosenberg, Vince Sabio, Hector Santos,   David F. Skoll, Paul Smith, and Brett Watson.Klensin                     Standards Track                    [Page 80]

RFC 5321                          SMTP                      October 2008   The efforts of the Area Directors -- Lisa Dusseault, Ted Hardie, and   Chris Newman -- to get this effort restarted and keep it moving, and   of an ad hoc committee with the same purpose, are gratefully   acknowledged.  The members of that committee were (in alphabetical   order) Dave Crocker, Cyrus Daboo, Tony Finch, Ned Freed, Randall   Gellens, Tony Hansen, the author, and Alexey Melnikov.  Tony Hansen   also acted as ad hoc chair on the mailing list reviewing this   document; without his efforts, sense of balance and fairness, and   patience, it clearly would not have been possible.10.  References10.1.  Normative References   [1]   Postel, J., "Simple Mail Transfer Protocol", STD 10,RFC 821,         August 1982.   [2]   Mockapetris, P., "Domain names - implementation and         specification", STD 13,RFC 1035, November 1987.   [3]   Braden, R., "Requirements for Internet Hosts - Application and         Support", STD 3,RFC 1123, October 1989.   [4]   Resnick, P., "Internet Message Format",RFC 5322, October 2008.   [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement         Levels",BCP 14,RFC 2119, March 1997.   [6]   American National Standards Institute (formerly United States         of America Standards Institute), "USA Code for Information         Interchange", ANSI X3.4-1968, 1968.         ANSI X3.4-1968 has been replaced by newer versions with slight         modifications, but the 1968 version remains definitive for the         Internet.   [7]   Crocker, D. and P. Overell, "Augmented BNF for Syntax         Specifications: ABNF", STD 68,RFC 5234, January 2008.   [8]   Hinden, R. and S. Deering, "IP Version 6 Addressing         Architecture",RFC 4291, February 2006.   [9]   Newman, C., "ESMTP and LMTP Transmission Types Registration",RFC 3848, July 2004.   [10]  Klensin, J., Freed, N., and K. Moore, "SMTP Service Extension         for Message Size Declaration", STD 10,RFC 1870, November 1995.Klensin                     Standards Track                    [Page 81]

RFC 5321                          SMTP                      October 2008   [11]  Klyne, G., Nottingham, M., and J. Mogul, "Registration         Procedures for Message Header Fields",BCP 90,RFC 3864,         September 2004.10.2.  Informative References   [12]  Partridge, C., "Mail routing and the domain system",RFC 974,         January 1986.   [13]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.         Crocker, "SMTP Service Extensions", STD 10,RFC 1869,         November 1995.   [14]  Klensin, J., "Simple Mail Transfer Protocol",RFC 2821,         April 2001.   [15]  Butler, M., Postel, J., Chase, D., Goldberger, J., and J.         Reynolds, "Post Office Protocol: Version 2",RFC 937,         February 1985.   [16]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",         STD 53,RFC 1939, May 1996.   [17]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION         4rev1",RFC 3501, March 2003.   [18]  Gellens, R. and J. Klensin, "Message Submission for Mail",RFC 4409, April 2006.   [19]  Freed, N., "SMTP Service Extension for Command Pipelining",         STD 60,RFC 2920, September 2000.   [20]  Vaudreuil, G., "SMTP Service Extensions for Transmission of         Large and Binary MIME Messages",RFC 3030, December 2000.   [21]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail         Extensions (MIME) Part One: Format of Internet Message Bodies",RFC 2045, November 1996.   [22]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.         Crocker, "SMTP Service Extension for 8bit-MIMEtransport",RFC 1652, July 1994.   [23]  Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part         Three: Message Header Extensions for Non-ASCII Text",RFC 2047,         November 1996.Klensin                     Standards Track                    [Page 82]

RFC 5321                          SMTP                      October 2008   [24]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word         Extensions: Character Sets, Languages, and Continuations",RFC 2231, November 1997.   [25]  Vaudreuil, G., "Enhanced Mail System Status Codes",RFC 3463,         January 2003.   [26]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced Mail         System Status Codes",BCP 138,RFC 5248, June 2008.   [27]  Freed, N., "Behavior of and Requirements for Internet         Firewalls",RFC 2979, October 2000.   [28]  Crocker, D., "Standard for the format of ARPA Internet text         messages", STD 11,RFC 822, August 1982.   [29]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for         Authorizing Use of Domains in E-Mail, Version 1",RFC 4408,         April 2006.   [30]  Fenton, J., "Analysis of Threats Motivating DomainKeys         Identified Mail (DKIM)",RFC 4686, September 2006.   [31]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and         M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures",RFC 4871, May 2007.   [32]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service         Extension for Delivery Status Notifications (DSNs)",RFC 3461,         January 2003.   [33]  Moore, K. and G. Vaudreuil, "An Extensible Message Format for         Delivery Status Notifications",RFC 3464, January 2003.   [34]  Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,RFC 959, October 1985.   [35]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping         between X.400 andRFC 822/MIME",RFC 2156, January 1998.   [36]  De Winter, J., "SMTP Service Extension for Remote Message Queue         Starting",RFC 1985, August 1996.   [37]  Hansen, T. and G. Vaudreuil, "Message Disposition         Notification",RFC 3798, May 2004.   [38]  Elz, R. and R. Bush, "Clarifications to the DNS Specification",RFC 2181, July 1997.Klensin                     Standards Track                    [Page 83]

RFC 5321                          SMTP                      October 2008   [39]  Nakamura, M. and J. Hagino, "SMTP Operational Experience in         Mixed IPv4/v6 Environments",RFC 3974, January 2005.   [40]  Partridge, C., "Duplicate messages and SMTP",RFC 1047,         February 1988.   [41]  Crispin, M., "Interactive Mail Access Protocol: Version 2",RFC 1176, August 1990.   [42]  Lambert, M., "PCMAIL: A distributed mail system for personal         computers",RFC 1056, June 1988.   [43]  Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security         Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",RFC 1847, October 1995.   [44]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.         Thayer, "OpenPGP Message Format",RFC 4880, November 2007.   [45]  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions         (S/MIME) Version 3.1 Message Specification",RFC 3851,         July 2004.   [46]  Internet Assigned Number Authority (IANA), "IANA Mail         Parameters", 2007,         <http://www.iana.org/assignments/mail-parameters>.   [47]  Internet Assigned Number Authority (IANA), "Address Literal         Tags", 2007,         <http://www.iana.org/assignments/address-literal-tags>.Klensin                     Standards Track                    [Page 84]

RFC 5321                          SMTP                      October 2008Appendix A.  TCP Transport Service   The TCP connection supports the transmission of 8-bit bytes.  The   SMTP data is 7-bit ASCII characters.  Each character is transmitted   as an 8-bit byte with the high-order bit cleared to zero.  Service   extensions may modify this rule to permit transmission of full 8-bit   data bytes as part of the message body, or, if specifically designed   to do so, in SMTP commands or responses.Appendix B.  Generating SMTP Commands fromRFC 822 Header Fields   Some systems use anRFC 822 header section (only) in a mail   submission protocol, or otherwise generate SMTP commands fromRFC 822   header fields when such a message is handed to an MTA from a UA.   While the MTA-UA protocol is a private matter, not covered by any   Internet Standard, there are problems with this approach.  For   example, there have been repeated problems with proper handling of   "bcc" copies and redistribution lists when information that   conceptually belongs to the mail envelope is not separated early in   processing from header field information (and kept separate).   It is recommended that the UA provide its initial ("submission   client") MTA with an envelope separate from the message itself.   However, if the envelope is not supplied, SMTP commands SHOULD be   generated as follows:   1.  Each recipient address from a TO, CC, or BCC header field SHOULD       be copied to a RCPT command (generating multiple message copies       if that is required for queuing or delivery).  This includes any       addresses listed in aRFC 822 "group".  Any BCC header fields       SHOULD then be removed from the header section.  Once this       process is completed, the remaining header fields SHOULD be       checked to verify that at least one TO, CC, or BCC header field       remains.  If none do, then a BCC header field with no additional       information SHOULD be inserted as specified in [4].   2.  The return address in the MAIL command SHOULD, if possible, be       derived from the system's identity for the submitting (local)       user, and the "From:" header field otherwise.  If there is a       system identity available, it SHOULD also be copied to the Sender       header field if it is different from the address in the From       header field.  (Any Sender header field that was already there       SHOULD be removed.)  Systems may provide a way for submitters to       override the envelope return address, but may want to restrict       its use to privileged users.  This will not prevent mail forgery,       but may lessen its incidence; seeSection 7.1.Klensin                     Standards Track                    [Page 85]

RFC 5321                          SMTP                      October 2008   When an MTA is being used in this way, it bears responsibility for   ensuring that the message being transmitted is valid.  The mechanisms   for checking that validity, and for handling (or returning) messages   that are not valid at the time of arrival, are part of the MUA-MTA   interface and not covered by this specification.   A submission protocol based on StandardRFC 822 information alone   MUST NOT be used to gateway a message from a foreign (non-SMTP) mail   system into an SMTP environment.  Additional information to construct   an envelope must come from some source in the other environment,   whether supplemental header fields or the foreign system's envelope.   Attempts to gateway messages using only their header "To" and "Cc"   fields have repeatedly caused mail loops and other behavior adverse   to the proper functioning of the Internet mail environment.  These   problems have been especially common when the message originates from   an Internet mailing list and is distributed into the foreign   environment using envelope information.  When these messages are then   processed by a header-section-only remailer, loops back to the   Internet environment (and the mailing list) are almost inevitable.Appendix C.  Source Routes   Historically, the <reverse-path> was a reverse source routing list of   hosts and a source mailbox.  The first host in the <reverse-path> was   historically the host sending the MAIL command; today, source routes   SHOULD NOT appear in the reverse-path.  Similarly, the <forward-path>   may be a source routing lists of hosts and a destination mailbox.   However, in general, the <forward-path> SHOULD contain only a mailbox   and domain name, relying on the domain name system to supply routing   information if required.  The use of source routes is deprecated (seeAppendix F.2); while servers MUST be prepared to receive and handle   them as discussed inSection 3.3 andAppendix F.2, clients SHOULD NOT   transmit them and this section is included in the current   specification only to provide context.  It has been modified somewhat   from the material inRFC 821 to prevent server actions that might   confuse clients or subsequent servers that do not expect a full   source route implementation.   For relay purposes, the forward-path may be a source route of the   form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST be fully-   qualified domain names.  This form is used to emphasize the   distinction between an address and a route.  The mailbox (here, JOE@   THREE) is an absolute address, and the route is information about how   to get there.  The two concepts should not be confused.   If source routes are used,RFC 821 and the text below should be   consulted for the mechanisms for constructing and updating theKlensin                     Standards Track                    [Page 86]

RFC 5321                          SMTP                      October 2008   forward-path.  A server that is reached by means of a source route   (e.g., its domain name appears first in the list in the forward-path)   MUST remove its domain name from any forward-paths in which that   domain name appears before forwarding the message and MAY remove all   other source routing information.  The reverse-path SHOULD NOT be   updated by servers conforming to this specification.   Notice that the forward-path and reverse-path appear in the SMTP   commands and replies, but not necessarily in the message.  That is,   there is no need for these paths and especially this syntax to appear   in the "To:" , "From:", "CC:", etc. fields of the message header   section.  Conversely, SMTP servers MUST NOT derive final message   routing information from message header fields.   When the list of hosts is present despite the recommendations above,   it is a "reverse" source route and indicates that the mail was   relayed through each host on the list (the first host in the list was   the most recent relay).  This list is used as a source route to   return non-delivery notices to the sender.  If, contrary to the   recommendations here, a relay host adds itself to the beginning of   the list, it MUST use its name as known in the transport environment   to which it is relaying the mail rather than that of the transport   environment from which the mail came (if they are different).  Note   that a situation could easily arise in which some relay hosts add   their names to the reverse source route and others do not, generating   discontinuities in the routing list.  This is another reason why   servers needing to return a message SHOULD ignore the source route   entirely and simply use the domain as specified in the Mailbox.Appendix D.  Scenarios   This section presents complete scenarios of several types of SMTP   sessions.  In the examples, "C:" indicates what is said by the SMTP   client, and "S:" indicates what is said by the SMTP server.Klensin                     Standards Track                    [Page 87]

RFC 5321                          SMTP                      October 2008D.1.  A Typical SMTP Transaction Scenario   This SMTP example shows mail sent by Smith at host bar.com, and to   Jones, Green, and Brown at host foo.com.  Here we assume that host   bar.com contacts host foo.com directly.  The mail is accepted for   Jones and Brown.  Green does not have a mailbox at host foo.com.      S: 220 foo.com Simple Mail Transfer Service Ready      C: EHLO bar.com      S: 250-foo.com greets bar.com      S: 250-8BITMIME      S: 250-SIZE      S: 250-DSN      S: 250 HELP      C: MAIL FROM:<Smith@bar.com>      S: 250 OK      C: RCPT TO:<Jones@foo.com>      S: 250 OK      C: RCPT TO:<Green@foo.com>      S: 550 No such user here      C: RCPT TO:<Brown@foo.com>      S: 250 OK      C: DATA      S: 354 Start mail input; end with <CRLF>.<CRLF>      C: Blah blah blah...      C: ...etc. etc. etc.      C: .      S: 250 OK      C: QUIT      S: 221 foo.com Service closing transmission channelKlensin                     Standards Track                    [Page 88]

RFC 5321                          SMTP                      October 2008D.2.  Aborted SMTP Transaction Scenario      S: 220 foo.com Simple Mail Transfer Service Ready      C: EHLO bar.com      S: 250-foo.com greets bar.com      S: 250-8BITMIME      S: 250-SIZE      S: 250-DSN      S: 250 HELP      C: MAIL FROM:<Smith@bar.com>      S: 250 OK      C: RCPT TO:<Jones@foo.com>      S: 250 OK      C: RCPT TO:<Green@foo.com>      S: 550 No such user here      C: RSET      S: 250 OK      C: QUIT      S: 221 foo.com Service closing transmission channelKlensin                     Standards Track                    [Page 89]

RFC 5321                          SMTP                      October 2008D.3.  Relayed Mail Scenario   Step 1 -- Source Host to Relay Host   The source host performs a DNS lookup on XYZ.COM (the destination   address) and finds DNS MX records specifying xyz.com as the best   preference and foo.com as a lower preference.  It attempts to open a   connection to xyz.com and fails.  It then opens a connection to   foo.com, with the following dialogue:      S: 220 foo.com Simple Mail Transfer Service Ready      C: EHLO bar.com      S: 250-foo.com greets bar.com      S: 250-8BITMIME      S: 250-SIZE      S: 250-DSN      S: 250 HELP      C: MAIL FROM:<JQP@bar.com>      S: 250 OK      C: RCPT TO:<Jones@XYZ.COM>      S: 250 OK      C: DATA      S: 354 Start mail input; end with <CRLF>.<CRLF>      C: Date: Thu, 21 May 1998 05:33:29 -0700      C: From: John Q. Public <JQP@bar.com>      C: Subject: The Next Meeting of the Board      C: To: Jones@xyz.com      C:      C: Bill:      C: The next meeting of the board of directors will be      C: on Tuesday.      C: John.      C: .      S: 250 OK      C: QUIT      S: 221 foo.com Service closing transmission channelKlensin                     Standards Track                    [Page 90]

RFC 5321                          SMTP                      October 2008   Step 2 -- Relay Host to Destination Host   foo.com, having received the message, now does a DNS lookup on   xyz.com.  It finds the same set of MX records, but cannot use the one   that points to itself (or to any other host as a worse preference).   It tries to open a connection to xyz.com itself and succeeds.  Then   we have:           S: 220 xyz.com Simple Mail Transfer Service Ready           C: EHLO foo.com           S: 250 xyz.com is on the air           C: MAIL FROM:<JQP@bar.com>           S: 250 OK           C: RCPT TO:<Jones@XYZ.COM>           S: 250 OK           C: DATA           S: 354 Start mail input; end with <CRLF>.<CRLF>           C: Received: from bar.com by foo.com ; Thu, 21 May 1998           C:     05:33:29 -0700           C: Date: Thu, 21 May 1998 05:33:22 -0700           C: From: John Q. Public <JQP@bar.com>           C: Subject:  The Next Meeting of the Board           C: To: Jones@xyz.com           C:           C: Bill:           C: The next meeting of the board of directors will be           C: on Tuesday.           C:                         John.           C: .           S: 250 OK           C: QUIT           S: 221 foo.com Service closing transmission channelKlensin                     Standards Track                    [Page 91]

RFC 5321                          SMTP                      October 2008D.4.  Verifying and Sending Scenario      S: 220 foo.com Simple Mail Transfer Service Ready      C: EHLO bar.com      S: 250-foo.com greets bar.com      S: 250-8BITMIME      S: 250-SIZE      S: 250-DSN      S: 250-VRFY      S: 250 HELP      C: VRFY Crispin      S: 250 Mark Crispin <Admin.MRC@foo.com>      C: MAIL FROM:<EAK@bar.com>      S: 250 OK      C: RCPT TO:<Admin.MRC@foo.com>      S: 250 OK      C: DATA      S: 354 Start mail input; end with <CRLF>.<CRLF>      C: Blah blah blah...      C: ...etc. etc. etc.      C: .      S: 250 OK      C: QUIT      S: 221 foo.com Service closing transmission channelAppendix E.  Other Gateway Issues   In general, gateways between the Internet and other mail systems   SHOULD attempt to preserve any layering semantics across the   boundaries between the two mail systems involved.  Gateway-   translation approaches that attempt to take shortcuts by mapping   (such as mapping envelope information from one system to the message   header section or body of another) have generally proven to be   inadequate in important ways.  Systems translating between   environments that do not support both envelopes and a header section   and Internet mail must be written with the understanding that some   information loss is almost inevitable.Klensin                     Standards Track                    [Page 92]

RFC 5321                          SMTP                      October 2008Appendix F.  Deprecated Features ofRFC 821   A few features ofRFC 821 have proven to be problematic and SHOULD   NOT be used in Internet mail.F.1.  TURN   This command, described inRFC 821, raises important security issues   since, in the absence of strong authentication of the host requesting   that the client and server switch roles, it can easily be used to   divert mail from its correct destination.  Its use is deprecated;   SMTP systems SHOULD NOT use it unless the server can authenticate the   client.F.2.  Source RoutingRFC 821 utilized the concept of explicit source routing to get mail   from one host to another via a series of relays.  The requirement to   utilize source routes in regular mail traffic was eliminated by the   introduction of the domain name system "MX" record and the last   significant justification for them was eliminated by the   introduction, inRFC 1123, of a clear requirement that addresses   following an "@" must all be fully-qualified domain names.   Consequently, the only remaining justifications for the use of source   routes are support for very old SMTP clients or MUAs and in mail   system debugging.  They can, however, still be useful in the latter   circumstance and for routing mail around serious, but temporary,   problems such as problems with the relevant DNS records.   SMTP servers MUST continue to accept source route syntax as specified   in the main body of this document and inRFC 1123.  They MAY, if   necessary, ignore the routes and utilize only the target domain in   the address.  If they do utilize the source route, the message MUST   be sent to the first domain shown in the address.  In particular, a   server MUST NOT guess at shortcuts within the source route.   Clients SHOULD NOT utilize explicit source routing except under   unusual circumstances, such as debugging or potentially relaying   around firewall or mail system configuration errors.F.3.  HELO   As discussed in Sections3.1 and4.1.1, EHLO SHOULD be used rather   than HELO when the server will accept the former.  Servers MUST   continue to accept and process HELO in order to support older   clients.Klensin                     Standards Track                    [Page 93]

RFC 5321                          SMTP                      October 2008F.4.  #-literalsRFC 821 provided for specifying an Internet address as a decimal   integer host number prefixed by a pound sign, "#".  In practice, that   form has been obsolete since the introduction of TCP/IP.  It is   deprecated and MUST NOT be used.F.5.  Dates and Years   When dates are inserted into messages by SMTP clients or servers   (e.g., in trace header fields), four-digit years MUST BE used.  Two-   digit years are deprecated; three-digit years were never permitted in   the Internet mail system.F.6.  Sending versus Mailing   In addition to specifying a mechanism for delivering messages to   user's mailboxes,RFC 821 provided additional, optional, commands to   deliver messages directly to the user's terminal screen.  These   commands (SEND, SAML, SOML) were rarely implemented, and changes in   workstation technology and the introduction of other protocols may   have rendered them obsolete even where they are implemented.   Clients SHOULD NOT provide SEND, SAML, or SOML as services.  Servers   MAY implement them.  If they are implemented by servers, the   implementation model specified inRFC 821 MUST be used and the   command names MUST be published in the response to the EHLO command.Author's Address   John C. Klensin   1770 Massachusetts Ave, Suite 322   Cambridge, MA  02140   USA   EMail: john+smtp@jck.comKlensin                     Standards Track                    [Page 94]

RFC 5321                          SMTP                      October 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Klensin                     Standards Track                    [Page 95]