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Network Working Group                               J. Klensin, WG ChairRequest for Comments: 1651                                           MCIObsoletes:1425                                         N. Freed, EditorCategory: Standards Track                                       Innosoft                                                                 M. Rose                                            Dover Beach Consulting, Inc.                                                            E. Stefferud                                     Network Management Associates, Inc.                                                              D. Crocker                                                  Silicon Graphics, Inc.                                                               July 1994SMTP Service ExtensionsStatus 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.1.  Abstract   This memo defines a framework for extending the SMTP service by   defining a means whereby a server SMTP can inform a client SMTP as to   the service extensions it supports. Standard extensions to the SMTP   service are registered with the IANA.  This framework does not   require modification of existing SMTP clients or servers unless the   features of the service extensions are to be requested or provided.2.  Introduction   The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable,   effective basis for the relay function of message transfer agents.   Although a decade old, SMTP has proven remarkably resilient.   Nevertheless, the need for a number of protocol extensions has become   evident. Rather than describing these extensions as separate and   haphazard entities, this document enhances SMTP in a straightforward   fashion that provides a framework in which all future extensions can   be built in a single consistent way.3.  Framework for SMTP Extensions   For the purpose of service extensions to SMTP, SMTP relays a mail   object containing an envelope and a content.Klensin, Freed, Rose, Stefferud & Crocker                       [Page 1]

RFC 1651                SMTP Service Extensions                July 1994   (1)  The SMTP envelope is straightforward, and is sent as a        series of SMTP protocol units: it consists of an        originator address (to which error reports should be        directed); a delivery mode (e.g., deliver to recipient        mailboxes); and, one or more recipient addresses.   (2)  The SMTP content is sent in the SMTP DATA protocol unit        and has two parts: the headers and the body. The headers        form a collection of field/value pairs structured        according to STD 11,RFC 822 [2], whilst the body, if        structured, is defined according to MIME [3]. The content is        textual in nature, expressed using the US-ASCII repertoire (ANSI        X3.4-1986). Although extensions (such as MIME) may relax        this restriction for the content body, the content        headers are always encoded using the US-ASCII repertoire.        The algorithm defined in [4] is used to represent header        values outside the US-ASCII repertoire, whilst still        encoding them using the US-ASCII repertoire.   Although SMTP is widely and robustly deployed, some parts of the   Internet community might wish to extend the SMTP service.  This memo   defines a means whereby both an extended SMTP client and server may   recognize each other as such and the server can inform the client as   to the service extensions that it supports.   It must be emphasized that any extension to the SMTP service should   not be considered lightly. SMTP's strength comes primarily from its   simplicity.  Experience with many protocols has shown that:     protocols with few options tend towards ubiquity, whilst     protocols with many options tend towards obscurity.   This means that 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.   Given this environment, the framework for the extensions described in   this memo consists of:      (1)  a new SMTP command (section 4)      (2)  a registry of SMTP service extensions (section 5)      (3)  additional parameters to the SMTP MAIL FROM and RCPT TO           commands (section 6).Klensin, Freed, Rose, Stefferud & Crocker                       [Page 2]

RFC 1651                SMTP Service Extensions                July 19944.  The EHLO command   A client SMTP supporting SMTP service extensions should start an SMTP   session by issuing the EHLO command instead of the HELO command. If   the SMTP server supports the SMTP service extensions it will give a   successful response (seesection 4.3), a failure response (see 4.4),   or an error response (4.5). If the SMTP server does not support any   SMTP service extensions it will generate an error response (seesection 4.5).4.1.  Changes to STD 10,RFC 821   STD 10,RFC 821 states that the first command in an SMTP session must   be the HELO command. This requirement is hereby amended to allow a   session to start with either EHLO or HELO.4.2.  Command syntax   The syntax for this command, using the ABNF notation of [2], is:     ehlo-cmd ::= "EHLO" SP domain CR LF   If successful, the server SMTP responds with code 250. On failure,   the server SMTP responds with code 550. On error, the server SMTP   responds with one of codes 500, 501, 502, 504, or 421.   This command is issued instead of the HELO command, and may be issued   at any time that a HELO command would be appropriate.  That is, if   the EHLO command is issued, and a successful response is returned,   then a subsequent HELO or EHLO command will result in the server SMTP   replying with code 503.  A client SMTP must not cache any information   returned if the EHLO command succeeds. That is, a client SMTP must   issue the EHLO command at the start of each SMTP session if   information about extended facilities is needed.4.3.  Successful response   If the server SMTP implements and is able to perform the EHLO   command, it will return code 250.  This indicates that both the   server and client SMTP are in the initial state, that is, there is no   transaction in progress and all state tables and buffers are cleared.   Normally, this response will be a multiline reply. Each line of the   response contains a keyword and, optionally, one or more parameters.   The syntax for a positive response, using the ABNF notation of [2],   is:Klensin, Freed, Rose, Stefferud & Crocker                       [Page 3]

RFC 1651                SMTP Service Extensions                July 1994     ehlo-ok-rsp  ::=      "250"    domain [ SP greeting ] CR LF                    / (    "250-"   domain [ SP greeting ] CR LF                        *( "250-"      ehlo-line           CR LF )                           "250"    SP ehlo-line           CR LF   )                  ; the usual HELO chit-chat     greeting     ::= 1*<any character other than CR or LF>     ehlo-line    ::= ehlo-keyword *( SP ehlo-param )     ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")                  ; syntax and values depend on ehlo-keyword     ehlo-param   ::= 1*<any CHAR excluding SP and all                         control characters (US-ASCII 0-31                         inclusive)>     ALPHA        ::= <any one of the 52 alphabetic characters                       (A through Z in upper case, and,                        a through z in lower case)>     DIGIT        ::= <any one of the 10 numeric characters                       (0 through 9)>     CR           ::= <the carriage-return character                       (ASCII decimal code 13)>     LF           ::= <the line-feed character                       (ASCII decimal code 10)>     SP           ::= <the space character                       (ASCII decimal code 32)>   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 begun inRFC 821.   The IANA maintains a registry of standard SMTP service extensions.   Associated with each such extension is a corresponding EHLO keyword   value. Each service extension registered with the IANA is defined by   a standards-track RFC, and such a definition includes:      (1)  the textual name of the SMTP service extension;      (2)  the EHLO keyword value associated with the extension;      (3)  the syntax and possible values of parameters associated           with the EHLO keyword value;Klensin, Freed, Rose, Stefferud & Crocker                       [Page 4]

RFC 1651                SMTP Service Extensions                July 1994      (4)  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);      (5)  any new parameters the extension associates with the MAIL           FROM or RCPT TO verbs; and,      (6)  how support for the extension affects the behavior of a           server and client SMTP.   In addition, any EHLO keyword value that starts with an upper or   lower case "X" refers to a local SMTP service extension, which is   used through bilateral, rather than standardized, agreement. Keywords   beginning with "X" may not be used in a registered service extension.   Any keyword values presented in the EHLO response that do not begin   with "X" must correspond to a standard or standards-track SMTP   service extension registered with IANA.  A conforming server must not   offer non "X" prefixed keyword values that are not described in a   registered and standardized extension.   Additional verbs are bound by the same rules as EHLO keywords;   specifically, verbs begining with "X" are local extensions that may   not be standardized and verbs not beginning with "X" must always be   registered.4.4.  Failure response   If for some reason the server SMTP is unable to list the service   extensions it supports, it will return code 554.   In the case of a failure response, the client SMTP should issue   either the HELO or QUIT command.4.5.  Error responses from extended servers   If the server SMTP recognizes the EHLO command, but the command   argument is unacceptable, it will return code 501.   If the server SMTP recognizes, but does not implement, the EHLO   command, it will return code 502.   If the server SMTP determines that the SMTP service is no longer   available (e.g., due to imminent system shutdown), it will return   code 421.   In the case of any error response, the client SMTP should issue   either the HELO or QUIT command.Klensin, Freed, Rose, Stefferud & Crocker                       [Page 5]

RFC 1651                SMTP Service Extensions                July 19944.6.  Responses from servers without extensions   A server SMTP that conforms toRFC 821 but does not support the   extensions specified here will not recognize the EHLO command and   will consequently return code 500, as specified inRFC 821.  The   server SMTP should stay in the same state after returning this code   (seesection 4.1.1 of RFC 821).  The client SMTP may then issue   either a HELO or a QUIT command.4.7.  Responses from improperly implemented servers   Some SMTP servers are known to disconnect the SMTP transmission   channel upon receipt of the EHLO command. The disconnect can occur   immediately or after sending a response.  Such behavior violatessection 4.1.1 of RFC 821, which explicitly states that disconnection   should only occur after a QUIT command is issued.   Nevertheless, in order to achieve maxmimum interoperablity it is   suggested that extended SMTP clients using EHLO be coded to check for   server connection closure after EHLO is sent, either before or after   returning a reply.  If this happens the client must decide if the   operation can be successfully completed without using any SMTP   extensions. If it can a new connection can be opened and the HELO   command can be used.   Other improperly-implemented servers will not accept a HELO command   after EHLO has been sent and rejected.  In some cases, this problem   can be worked around by sending a RSET after the failure response to   EHLO, then sending the HELO.  Clients that do this should be aware   that many implementations will return a failure code (e.g., 503 Bad   sequence of commands) in response to the RSET.  This code can be   safely ignored.5.  Initial IANA Registry   The IANA's initial registry of SMTP service extensions consists of   these entries:   Service Ext   EHLO Keyword Parameters Verb       Added Behavior   ------------- ------------ ---------- ---------- ------------------   Send             SEND         none       SEND    defined inRFC 821   Send or Mail     SOML         none       SOML    defined inRFC 821   Send and Mail    SAML         none       SAML    defined inRFC 821   Expand           EXPN         none       EXPN    defined inRFC 821   Help             HELP         none       HELP    defined inRFC 821   Turn             TURN         none       TURN    defined inRFC 821Klensin, Freed, Rose, Stefferud & Crocker                       [Page 6]

RFC 1651                SMTP Service Extensions                July 1994   which correspond to those SMTP commands which are defined as optional   in [5].  (The mandatory SMTP commands, according to [5], are HELO,   MAIL, RCPT, DATA, RSET, VRFY, NOOP, and QUIT.)6.  MAIL FROM and RCPT TO Parameters   It is recognized that several of the extensions planned for SMTP will   make use of additional parameters associated with the MAIL FROM and   RCPT TO command. The syntax for these commands, again using the ABNF   notation of [2] as well as underlying definitions from [1], is:  esmtp-cmd        ::= inner-esmtp-cmd [SP esmtp-parameters] CR LF  esmtp-parameters ::= esmtp-parameter *(SP esmtp-parameter)  esmtp-parameter  ::= esmtp-keyword ["=" esmtp-value]  esmtp-keyword    ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")                       ; syntax and values depend on esmtp-keyword  esmtp-value      ::= 1*<any CHAR excluding "=", SP, and all                          control characters (US-ASCII 0-31                          inclusive)>                       ; The following commands are extended to                       ; accept extended parameters.  inner-esmtp-cmd  ::= ("MAIL FROM:<" reverse-path ">")   /                       ("RCPT TO:<" forward-path ">")   All esmtp-keyword values must be registered as part of the IANA   registration process described above. This definition only provides   the framework for future extension; no extended MAIL FROM or RCPT TO   parameters are defined by this RFC.6.1.  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 accomodate 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.   Errors specific to particular parameters and their values will be   specified in the parameter's defining RFC.Klensin, Freed, Rose, Stefferud & Crocker                       [Page 7]

RFC 1651                SMTP Service Extensions                July 19947.  Received: Header Field Annotation   SMTP servers are required to add an appropriate Received: field to   the headers of all messages they receive. A "with ESMTP" clause   should be added to this field when any SMTP service extensions are   used. "ESMTP" is hereby added to the list of standard protocol names   registered with IANA.8.  Usage Examples   (1)  An interaction of the form:        S: <wait for connection on TCP port 25>        C: <open connection to server>        S: 220 dbc.mtview.ca.us SMTP service ready        C: EHLO ymir.claremont.edu        S: 250 dbc.mtview.ca.us says hello         ...        indicates that the server SMTP implements only those SMTP        commands which are defined as mandatory in [5].   (2)  In contrast, an interaction of the form:        S: <wait for connection on TCP port 25>        C: <open connection to server>        S: 220 dbc.mtview.ca.us SMTP service ready        C: EHLO ymir.claremont.edu        S: 250-dbc.mtview.ca.us says hello        S: 250-EXPN        S: 250-HELP        S: 250-8BITMIME        S: 250-XONE        S: 250 XVRB         ...        indicates that the server SMTP also implements the SMTP        EXPN and HELP commands, one standard service extension        (8BITMIME), and two non-standard service extensions (XONE        and XVRB).   (3)  Finally, a server that does not support SMTP service        extensions would act as follows:        S: <wait for connection on TCP port 25>        C: <open connection to server>Klensin, Freed, Rose, Stefferud & Crocker                       [Page 8]

RFC 1651                SMTP Service Extensions                July 1994        S: 220 dbc.mtview.ca.us SMTP service ready        C: EHLO ymir.claremont.edu        S: 500 Command not recognized: EHLO         ...        The 500 response indicates that the server SMTP does not        implement the extensions specified here.  The client        would normally send a HELO command and proceed as        specified inRFC 821.   Seesection 4.7 for additional        discussion.9.  Security Considerations   This RFC does not discuss security issues and is not believed to   raise any security issues not already endemic in electronic mail and   present in fully conforming implementations ofRFC-821.  It does   provide an announcement of server mail capabilities via the response   to the EHLO verb. However, all information provided by announcement   of any of the initial set of service extensions defined by this RFC   can be readily deduced by selective probing of the verbs required to   transport and deliver mail. The security implications of service   extensions described in other RFCs should be dealt with in those   RFCs.10.  Acknowledgements   This document represents a synthesis of the ideas of many people and   reactions to the ideas and proposals of others.  Randall Atkinson,   Craig Everhart, Risto Kankkunen, and Greg Vaudreuil contributed ideas   and text sufficient to be considered co-authors.  Other important   suggestions, text, or encouragement came from Harald Alvestrand, Jim   Conklin, Mark Crispin, Frank da Cruz, 'Olafur Gudmundsson, Per   Hedeland, Christian Huitma, Neil Katin, Eliot Lear, Harold A.   Miller, Dan Oscarsson, Julian Onions, Rayan Zachariassen, and the   contributions of the entire IETF SMTP Working Group. Of course, none   of the individuals are necessarily responsible for the combination of   ideas represented here. Indeed, in some cases, the response to a   particular criticism was to accept the problem identification but to   include an entirely different solution from the one originally   proposed.11.  References   [1] Postel, J., "Simple Mail Transfer Protocol", STD 10,RFC 821,       USC/Information Sciences Institute, August 1982.   [2] Crocker, D., "Standard for the Format of ARPA Internet Text       Messages", STD 11,RFC 822, UDEL, August 1982.Klensin, Freed, Rose, Stefferud & Crocker                       [Page 9]

RFC 1651                SMTP Service Extensions                July 1994   [3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail       Extensions",RFC 1521, Bellcore, Innosoft, September 1993.   [4] Moore, K., "Representation of Non-ASCII Text in Internet Message       Headers",RFC 1522, University of Tennessee, September 1993.   [5] Braden, R., "Requirements for Internet Hosts - Application and       Support", STD 3,RFC 1123, USC/Information Sciences Institute,       October 1989.12.  Chair, Editor, and Authors' Addresses   John Klensin, WG Chair   MCI Data Services Division   2100 Reston Parkway, 6th floor   Reston, VA 22091   USA   Phone:: 1 703 715 7361   Fax: +1 703 715 7435   EMail: klensin@mci.net   Ned Freed, Editor   Innosoft International, Inc.   1050 East Garvey Avenue South   West Covina, CA 91790   USA   Phone:: +1 818 919 3600   Fax: +1 818 919 3614   EMail: ned@innosoft.com   Marshall T. Rose   Dover Beach Consulting, Inc.   420 Whisman Court   Moutain View, CA  94043-2186   USA   Phone: +1 415 968 1052   Fax: +1 415 968 2510   EMail: mrose@dbc.mtview.ca.usKlensin, Freed, Rose, Stefferud & Crocker                      [Page 10]

RFC 1651                SMTP Service Extensions                July 1994   Einar A. Stefferud   Network Management Associates, Inc.   17301 Drey Lane   Huntington Beach, CA, 92647-5615   USA   Phone: +1 714 842 3711   Fax: +1 714 848 2091   EMail: stef@nma.com   Dave Crocker   Silicon Graphics, Inc.   2011 N. Shoreline Blvd.   P.O. Box 7311   Mountain View, CA 94039   USA   Phone: +1 415 390 1804   Fax: +1 415 962 8404   EMail: dcrocker@sgi.comKlensin, Freed, Rose, Stefferud & Crocker                      [Page 11]