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Network Working Group                                           K. MooreRequest for Comments: 3834                       University of TennesseeCategory: Standards Track                                    August 2004Recommendations for Automatic Responses to Electronic MailStatus 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.Copyright Notice   Copyright (C) The Internet Society (2004).Abstract   This memo makes recommendations for software that automatically   responds to incoming electronic mail messages, including "out of the   office" or "vacation" response generators, mail filtering software,   email-based information services, and other automatic responders.   The purpose of these recommendations is to discourage undesirable   behavior which is caused or aggravated by such software, to encourage   uniform behavior (where appropriate) among automatic mail responders,   and to clear up some sources of confusion among implementors of   automatic email responders.1.  Introduction   Many programs which automatically respond to email are currently in   use.  Although these programs vary widely in their function, several   problems with this class of programs have been observed, including:   significant numbers of useless or unwanted response and responses   sent to inappropriate addresses, and occasional incidences of mail   loops or "sorcerer's apprentice" mode.  This memo recommends behavior   for programs that automatically respond to electronic mail in order   to reduce the number of problems caused by such programs.   (Note: the term "sorcerer's apprentice mode" is defined as a bug in a   protocol where, under some circumstances, the receipt of a message   causes multiple messages to be sent, each of which, when received,   triggers the same bug.) (From [I1.JARGON])Moore                       Standards Track                     [Page 1]

RFC 3834               Automatic E-Mail Responses            August 2004   This document is limited in scope to Internet electronic mail   messages and many of its recommendations are specifically tailored   for the protocol elements and data models used in Internet electronic   mail messages and SMTP transport envelopes.  Use of these   recommendations in other messaging contexts such as instant   messaging, SMS, or Usenet has not been considered, and is outside of   the scope of this document.1.1.  Types of automatic responses   There are several different types of automatic responses.  At least   two types of automatic responses have been defined in IETF standards   - Delivery Status Notifications [I2.RFC3464] which are intended to   report the status of a message delivery by the message transport   system, and Message Disposition Notifications [I3.RFC3798] which are   intended to report of the disposition of a message after it reaches a   recipient's mailbox.  These responses are defined elsewhere and are   generally not within the purview of this document, except that this   document recommends specific cases where they should or should not be   used.   Other types of automatic response in common use include:   -  "Out of office" or "vacation" notices, which are intended to      inform the sender of a message that the message is unlikely to be      read, or acted on, for some amount of time,   -  "Change of address" notices, intended to inform the sender of a      message that the recipient address he used is obsolete and that a      different address should be used instead (whether or not the      subject message was forwarded to the current address),   -  "Challenges", which require the sender of a message to demonstrate      some measure of intelligence and/or willingness to agree to some      conditions before the subject message will be delivered to the      recipient (often to minimize the effect of "spam" or viruses on      the recipient),   -  Email-based information services, which accept requests      (presumably from humans) via email, provide some service, and      issue responses via email also.  (Mailing lists which accept      subscription requests via email fall into this category),   -  Information services similar to those mentioned above except that      they are intended to accept messages from other programs, andMoore                       Standards Track                     [Page 2]

RFC 3834               Automatic E-Mail Responses            August 2004   -  Various kinds of mail filters (including "virus scanners") which      act on behalf of a recipient to alter the content of messages      before forwarding them to that recipient, and issue responses in      the event a message is altered.   Recognizing the wide variety of response types in use, these   recommendations distinguish between several classes of automatic   responders according to the party or service on whose behalf the   responder acts:   -  "Service Responders" exist to provide access to some service via      email requests and responses.  These are permanently associated      with one or more email addresses, and when sending to such an      address the sender presumably expects an automatic response.  An      email-based file retrieval service is an example of a Service      Responder.  A calendar service that allows appointment requests to      be made via email, and which responds to such requests, would be      another example of a Service Responder.   -  "Personal Responders" exist to make automatic responses on behalf      of a single recipient address, in addition to, or in lieu of, that      recipient reading the message.  These responders operate according      to criteria specified on a per-recipient basis.  The UNIX      "vacation" program is an example of a Personal Responder.  A      responder that accepts mail sent to a single address, attempts to      analyze and classify the contents, and then issues a response      which is dependent on that classification, is also a Personal      Responder.   -  "Group Responders" exist to make automatic responses on behalf of      any of a significant set of recipient addresses (say, every      recipient in a particular DNS domain), in advance of, or in lieu      of, a response from the actual recipient.  Group Responders are      similar to Personal Responders except that in the case of a Group      Responder the criteria for responding are not set on a per-      recipient basis.  A "virus scanner" program that filtered all mail      sent to any recipient on a particular server, and sent responses      when a message was rejected or delivered in an altered form, might      be an example of a Group Responder.   Appropriate behavior for a responder varies from one class to   another.  A behavior which might be appropriate from a Service   Responder (where the sender is expecting an automatic response) might   not be appropriate from a Personal Responder.  For example, a Service   Responder might send a very long response to a request, or one that   is not in a human-readable format, according to the needs of thatMoore                       Standards Track                     [Page 3]

RFC 3834               Automatic E-Mail Responses            August 2004   service.  However a Personal Responder should assume that a human   being is reading the response and send only brief responses in plain   text.1.2.  Notation and Definitions   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",   "RECOMMENDED", "NOT RECOMMENDED", and "MAY" in this document are to   be interpreted as described in [N1.RFC2119].   The term "subject message" is used to refer to a message which causes   a response to be sent.   The term "response" refers to a message that is automatically issued   on receipt of a subject message by a responder.   A "responder" is a process that automatically responds to subject   messages under some well-defined set of conditions.   Unless specified otherwise, the term "recipient" refers to the email   addresses to which a subject message was delivered (rather than, for   instance, the address to which the response was sent).  A "recipient"   address might be permanently associated with a responder, or it might   be the address of a human being whose mail is, under some conditions,   answered by a responder.2.  When (not) to send automatic responses   An automatic responder MUST NOT blindly send a response for every   message received.  In practice there are always reasons to refuse to   respond to some kinds of received messages, e.g., for loop   prevention, to avoid responding to "spam" or viruses, to avoid being   used as a means to launder or amplify abusive messages, to avoid   inappropriately revealing personal information about the recipient   (e.g., to avoid an automatic indication that a recipient has not read   his mail recently), and to thwart denial-of-service attacks against   the responder.  The criteria for deciding whether to respond will   differ from one responder to another, according to the responder's   purpose.  In general, care should be taken to avoid sending useless   or redundant responses, and to avoid contributing to mail loops or   facilitating denial-of-service attacks.   Here are some broad guidelines:   -  Automatic responses SHOULD NOT be issued in response to any      message which contains an Auto-Submitted header field (see below),      where that field has any value other than "no".Moore                       Standards Track                     [Page 4]

RFC 3834               Automatic E-Mail Responses            August 2004   -  Personal and Group responses that are intended to notify the      sender of a message of the recipient's inability to read or reply      to the message (e.g., "away from my mail" or "too busy"      notifications) SHOULD NOT issue the same response to the same      sender more than once within a period of several days, even though      that sender may have sent multiple messages.  A 7-day period is      RECOMMENDED as a default.   -  Personal and Group responses whose purpose is to notify the sender      of a message of a temporary absence of the recipient (e.g.,      "vacation" and "out of the office" notices) SHOULD NOT be issued      unless a valid address for the recipient is explicitly included in      a recipient (e.g., To, Cc, Bcc, Resent-To, Resent-Cc, or Resent-      Bcc) field of the subject message.  Since a recipient may have      multiple addresses forwarded to the same mailbox, recipients      SHOULD be able to specify a set of addresses to the responder      which it will recognize as valid for that recipient.      Note:RFC 2822 section 3.6.3 permits varying uses of the Bcc      field, some of which would allow the sender of the subject message      to explicitly specify the recipient's address as a "Bcc" recipient      without a Bcc field appearing in the message as delivered, or      without the Bcc field in the delivered message containing the      recipient's address.  However, perhaps because Bcc's are rarely      used, the heuristic of not responding to messages for which the      recipient was not explicitly listed in a To, CC, or Bcc header      field has been found to work well in practice.   -  Personal and Group Responders MAY refuse to generate responses      except to known correspondents or addresses of otherwise "trusted"      individuals.  Such responders MAY also generate different kinds of      responses for "trusted" vs. "untrusted" addresses.  This might be      useful, for instance, to avoid inappropriate disclosure of      personal information to arbitrary addresses.   -  Responders MUST NOT generate any response for which the      destination of that response would be a null address (e.g., an      address for which SMTP MAIL FROM or Return-Path is <>), since the      response would not be delivered to a useful destination.      Responders MAY refuse to generate responses for addresses commonly      used as return addresses by responders - e.g., those with local-      parts matching "owner-*", "*-request", "MAILER-DAEMON", etc.      Responders are encouraged to check the destination address for      validity before generating the response, to avoid generating      responses that cannot be delivered or are unlikely to be useful.Moore                       Standards Track                     [Page 5]

RFC 3834               Automatic E-Mail Responses            August 2004   -  In order to avoid responding to spam and to certain kinds of      attacks, automatic responses from Service Responders SHOULD NOT be      sent for extremely malformed requests.  This may include checking      that the subject message has a content-type and content      appropriate to that service.   -  Because the vast majority of email is unauthenticated, and return      addresses are easily forged, in order to avoid being used as a      means of denial-of-service attacks (i.e., to flood mailboxes with      unwanted content) Service Responders SHOULD NOT return large      responses (say, more than a few kilobytes) without specific      knowledge that the request was actually authorized by the party      associated with the address to which the response will be sent.      Similarly, Service Responders SHOULD NOT cause unwanted side-      effects (such as subscribing the sender to a mailing list) without      reasonable assurance that the request was authorized by the      affected party.      NOTE: Since each responder has a different purpose and a different      set of potential threats to which it might be subjected, whether      any particular means of authentication is appropriate for a      particular responder is not in scope for this document.   -  A responder MAY refuse to send a response to a subject message      which contains any header or content which makes it appear to the      responder that a response would not be appropriate.  For instance,      if the subject message contained a Precedence header field      [I4.RFC2076] with a value of "list" the responder might guess that      the traffic had arrived from a mailing list, and would not respond      if the response were only intended for personal messages.  For      similar reasons, a responder MAY ignore any subject message with a      List-* field [I5.RFC2369].  (Because Precedence is not a standard      header field, and its use and interpretation vary widely in the      wild, no particular responder behavior in the presence of      Precedence is recommended by this specification.)3.  Format of automatic responses   The following sections specify details of the contents of automatic   responses, including the header of the response message, the content   of the response, and the envelope in which the response is   transmitted to the email transport system.Moore                       Standards Track                     [Page 6]

RFC 3834               Automatic E-Mail Responses            August 20043.1.  Message header   The fields in the message header should be set as follows:3.1.1.  From field   In correspondence between humans, the From field serves multiple   purposes: It identifies the author of the message (or in some cases,   the party or parties on whose behalf the message was sent), and it is   the default destination of replies from humans.  Unfortunately, some   mail systems still send non-delivery reports and other kinds of   automatic responses to the From address.   For automatic responses, the role of the From field in determining   the destination of replies to the response from humans is less   significant, because in most cases it is not useful or appropriate   for a human (or anyone) to reply to an automatic response.  One   exception is when there is some problem with the response; it should   be possible to provide feedback to the person operating the   responder.   So in most cases the From address in an automatic response needs to   be chosen according to the following criteria:   -  To provide an indication of the party or agent on whose behalf the      response was sent,   -  To provide an address to which a recipient of an inappropriate      response can request that the situation be corrected, and   -  To diminish the potential for mail loops.   The following behavior is thus recommended:   -  For responses sent by Service Responders, the From field SHOULD      contain an address which can be used to reach the (human)      maintainer of that service.  The human-readable portion of the      From field (the display-name preceding the address) SHOULD contain      a name or description of the service to identify the service to      humans.   -  For responses sent by Personal Responders, the From field SHOULD      contain the name of the recipient of the subject message (i.e.,      the user on whose behalf the response is being sent) and an      address chosen by the recipient of the subject message to be      recognizable to correspondents.  Often this will be the same      address that was used to send the subject message to that      recipient.Moore                       Standards Track                     [Page 7]

RFC 3834               Automatic E-Mail Responses            August 2004      In the case of a recipient having multiple mail addresses      forwarded to the same mailbox (and responder), a Personal      Responder MAY use heuristics to guess, based on the information      available in various message header fields, which of several      addresses for that recipient the sender is likely to have used,      and use that address in the From field of the response.  However      it MUST be possible for a recipient on whose behalf the responder      is acting to explicitly specify the human-readable name and      address to be used in the From header fields of responses.      Note: Due to privacy reasons it may be inappropriate for      responders to disclose an address that is derived, say, from the      recipient's login information (e.g., POP or IMAP user name or      account name on a multiuser computer) or which discloses the      specific name of the computer where the response was generated.      Furthermore these do not necessarily produce a valid public email      address for the recipient.  For this reason, Personal Responders      MUST allow the From field of a Personal Response to be set by the      recipient on whose behalf the responder is acting.   -  For Group Responders, the From address SHOULD contain an email      address which could be used to reach the maintainer of that Group      Responder.  Use of the Postmaster address for this purpose is NOT      RECOMMENDED.      The human-readable portion of the From address (the "phrase"      before the address, see [N2.RFC2822],section 3.2.6) SHOULD      contain an indication of the function performed by the Group      Responder and on whose behalf it operates (e.g., "Example Agency      virus filter")3.1.2.  Reply-To field   If a reply is expected by the responder, the Reply-To field of the   response SHOULD be set to the address at which the reply is expected,   even if this is the address of the same or another responder.   Responders which request replies to be sent to responders MUST   prevent mail loops and sorcerer's apprentice mode.  Note that since   (according to the previous section) the From field of the response   SHOULD contain the address of a human, if the Reply-To field of the   response is used to direct replies to a responder it will not be the   same as the address in the From field.   Discussion: this assumes that the human recipient's user agent will   normally send replies to the Reply-To address (if present), as   recommended by [I6.RFC822] since 1982, but that it is still possibleMoore                       Standards Track                     [Page 8]

RFC 3834               Automatic E-Mail Responses            August 2004   for a recipient to reply to the From address if he or she finds it   useful to do so.  This is consistent with the intended use of these   fields in [I6.RFC822] and [N2.RFC2822].3.1.3.  To field   The To header field SHOULD indicate the recipient of the response.   In general there SHOULD only be one recipient of any automatic   response.  This minimizes the potential for sorcerer's apprentice   mode and denial-of-service attacks.3.1.4.  Date field   The Date header field SHOULD indicate the date and time at which the   response was generated.  This MUST NOT be taken as any indication of   the delivery date of the subject message, nor of the time at which   the response was sent.3.1.5.  Subject field   The Subject field SHOULD contain a brief indication that the message   is an automatic response, followed by contents of the Subject field   (or a portion thereof) from the subject message.  The prefix "Auto:"   MAY be used as such an indication.  If used, this prefix SHOULD be   followed by an ASCII SPACE character (0x20).   NOTE: Just as the (Latin-derived) prefix "Re:" that is commonly used   to indicate human-generated responses is sometimes translated to   other languages by mail user agents, or otherwise interpreted by mail   user agents as indication that the message is a reply, so the (Greek)   prefix "Auto:" may also be translated or used as a generic indication   that the message is an automatic response.  However the "Auto:"   indication is intended only as an aid to humans in processing the   message.  Mail processing software SHOULD NOT assume that the   presence of "Auto:" at the beginning of a Subject field is an   indication that the message was automatically submitted.   Note that the Subject field of the subject message may contain   encoded-words formatted according to [N3.RFC2047] and [N4.RFC2231],   and such text MAY be included in the Subject field of a response.  In   generating responses containing such fields there is rarely a need to   decode and re-encode such text.  It is usually sufficient to leave   those encoded-words as they were in the subject message, merely   prepending "Auto: " or other indication.  However, it is still   necessary to ensure that no line in the resulting Subject field that   contains an encoded-word is greater than 76 ASCII characters in   length (this refers to the encoded form, not the number of charactersMoore                       Standards Track                     [Page 9]

RFC 3834               Automatic E-Mail Responses            August 2004   in the text being encoded).  Also, if the responder truncates the   Subject from the subject message it is necessary to avoid truncating   Subject text in the middle of an encoded-word.3.1.6.  In-Reply-To and References fields   The In-Reply-To and References fields SHOULD be provided in the   header of a response message if there was a Message-ID field in the   subject message, according to the rules in [N2.RFC2822]section3.6.4.3.1.7.  Auto-Submitted field   The Auto-Submitted field, with a value of "auto-replied", SHOULD be   included in the message header of any automatic response.  Seesection 5.3.1.8.  Precedence field   A response MAY include a Precedence field [I4.RFC2076] in order to   discourage responses from some kinds of responders which predate this   specification.  The field-body of the Precedence field MAY consist of   the text "junk", "list", "bulk", or other text deemed appropriate by   the responder.  Because the Precedence field is non-standard and its   interpretation varies widely, the use of Precedence is not   specifically recommended by this specification, nor does this   specification recommend any particular value for that field.3.2.  Message content   In general, messages sent by Personal or Group Responders SHOULD be   brief, and in text/plain format.  A multipart/alternative construct   MAY be used to communicate responses in multiple languages,   especially if in doing so it is desirable to use multiple charsets.   Response messages SHOULD NOT include significant content from the   subject message.  In particular, Personal and Group responses SHOULD   NOT contain non-text content from the subject message, and they   SHOULD NOT include attachments from the subject message.  Neither of   these conditions applies to responders that specifically exist for   the purpose of altering or translating content sent to them (for   instance, a FORTRAN-to-C translator); however, such responders MUST   employ measures to avoid being used as a means of laundering or   forwarding undesirable content, such as spam or viruses.   Note that when text from the Subject or other fields from the header   of the subject message is included in the body of the response, it is   necessary to decode any encoded-words that appeared in those fieldsMoore                       Standards Track                    [Page 10]

RFC 3834               Automatic E-Mail Responses            August 2004   before including in the message body, and to use an appropriate   content-type, charset, and content-transfer-encoding.  In some cases   it may be necessary to transliterate text from the charset(s) used in   the header of the subject message, to the charset(s) used in the body   of the response.  (It is much easier to implement a responder if text   from the header of the subject message never needs to appear in the   body of the response.)3.2.1.  Use of DSNs and MDNs instead of this specification   In general, it is appropriate to use Delivery Status Notifications   (DSNs) for responses that are generated by the mail transport system   as a result of attempts to relay, forward, or deliver mail, and only   when the purpose of that response is to provide the sender of the   subject message with information about the status of that mail   delivery.  For instance, a "virus scanner" which is activated by a   mail delivery process to filter harmful content prior to delivery,   could return a DSN with the Action field set to "failed" with a   Status code of 5.7.1 (Delivery not authorized, message refused) if   the entire message was not delivered due to security reasons; or it   could return a DSN with the Action field set to "relayed" or   "delivered" (as appropriate) with a Status code set to 2.6.4   (conversion with loss performed) if the message was relayed or   delivered with the presumably harmful content removed.  The DSN   specification [I2.RFC3464], rather than this document, governs the   generation and format of DSNs.   Similarly, it is appropriate to use Message Disposition Notifications   (MDNs) only for responses generated on the recipient's behalf, which   are generated on or after delivery to a recipient's mailbox, and for   which the purpose of the response is to indicate the disposition of   the message.  The MDN specification [I3.RFC3798], rather than this   document, governs the generation and format of MDNs.   This document is not intended to alter either the DSN or MDN   specifications.  Responses that fit within the criteria of DSN or   MDN, as defined by the respective specifications, should be generated   according to the DSN or MDN specification rather than this document.   Responses which do not fit one of these sets of criteria should be   generated according to this document.3.3.  Message envelope   The SMTP MAIL FROM address, or other envelope return address used to   send the message, SHOULD be chosen in such a way as to make mail   loops unlikely.  A loop might occur, for instance, if both sender andMoore                       Standards Track                    [Page 11]

RFC 3834               Automatic E-Mail Responses            August 2004   recipient of a message each have automatic responders - the   recipient's responder sends mail to the sender's responder, which   sends mail back to the recipient's responder.   The primary purpose of the MAIL FROM address is to serve as the   destination for delivery status messages and other automatic   responses.  Since in most cases it is not appropriate to respond to   an automatic response, and the responder is not interested in   delivery status messages, a MAIL FROM address of <> MAY be used for   this purpose.  A MAIL FROM address which is specifically chosen for   the purpose of sending automatic responses, and which will not   automatically respond to any message sent to it, MAY be used instead   of <>.   The RCPT TO address will (of course) be the address of the intended   recipient of the response.  It is RECOMMENDED that the NOTIFY=NEVER   parameter of the RCPT command be specified if the SMTP server   supports the DSN option [N5.RFC3461].4.  Where to send automatic responses (and where not to send them)   In general, automatic responses SHOULD be sent to the Return-Path   field if generated after delivery.  If the response is generated   prior to delivery, the response SHOULD be sent to the reverse-path   from the SMTP MAIL FROM command, or (in a non-SMTP system) to the   envelope return address which serves as the destination for non-   delivery reports.   If the response is to be generated after delivery, and there is no   Return-Path field in the subject message, there is an implementation   or configuration error in the SMTP server that delivered the message   or gatewayed the message outside of SMTP.  A Personal or Group   responder SHOULD NOT deliver a response to any address other than   that in the Return-Path field, even if the Return-Path field is   missing.  It is better to fix the problem with the mail delivery   system than to rely on heuristics to guess the appropriate   destination of the response.  Such heuristics have been known to   cause problems in the past.   A Service Responder MAY deliver the response to the address(es) from   the >From field, or to another address from the request payload,   provided this behavior is precisely defined in the specification for   that service.  Services responders SHOULD NOT use the Reply-To field   for this purpose.   The Reply-To field SHOULD NOT be used as the destination for   automatic responses from Personal or Group Responders.  In general,   this field is set by a human sender based on his/her anticipation ofMoore                       Standards Track                    [Page 12]

RFC 3834               Automatic E-Mail Responses            August 2004   how human recipients will respond to the specific content of that   message.  For instance, a human sender may use Reply-To to request   that replies be sent to an entire mailing list.  Even for replies   from humans, there are cases where it is not appropriate to respond   to the Reply-To address, especially if the sender has asked that   replies be sent to a group and/or mailing list.  Since a Personal or   Group Responder operates on behalf of a human recipient, it is safer   to assume that any Reply-To field present in the message was set by a   human sender on the assumption that any reply would come from a human   who had some understanding of the roles of the sender and other   recipients.  An automatic responder lacks the information necessary   to understand those roles.  Sending automatic responses to Reply-To   addresses can thus result in a large number of people receiving a   useless or unwanted message; it can also contribute to mail loops.   Use of the From field as the destination for automatic responses has   some of the same problems as use of Reply-To.  In particular, the   From field may list multiple addresses, while automatic responses   should only be sent to a single address.  In general, the From and   Reply-To addresses are used in a variety of ways according to   differing circumstances, and for this reason Personal or Group   Responders cannot reliably assume that an address in the From or   Reply-To field is an appropriate destination for the response.  For   these reasons the From field SHOULD NOT be used as a destination for   automatic responses.   Similarly, the Sender field SHOULD NOT be used as the destination for   automatic responses.  This field is intended only to identify the   person or entity that sent the message, and is not required to   contain an address that is valid for replies.   The Return-Path address is really the only one from the message   header that can be expected, as a matter of protocol, to be suitable   for automatic responses that were not anticipated by the sender.5.  The Auto-Submitted header field   The purpose of the Auto-Submitted header field is to indicate that   the message was originated by an automatic process, or an automatic   responder, rather than by a human; and to facilitate automatic   filtering of messages from signal paths for which automatically   generated messages and automatic responses are not desirable.5.1.  Syntax   The syntax of Auto-Submitted is as follows, using the ABNF notation   of [N6.RFC2234]:Moore                       Standards Track                    [Page 13]

RFC 3834               Automatic E-Mail Responses            August 2004   auto-submitted-field     = "Auto-Submitted:" [CFWS]                              auto-submitted [CFWS] CRLF   auto-submitted           = ( "no" / "auto-generated" /                              "auto-replied" / extension )                              opt-parameter-list   extension                = token   opt-parameter-list       = *( [CFWS] ";" [CFWS] parameter )   The symbols "CFWS" and "CRLF" are defined in [N2.RFC2822].  The   symbols "token", and "parameter" are as defined in [N7.RFC2045] (as   amended by [N4.RFC2231]).   The maximum number of Auto-Submitted fields that may appear in a   message header is 1.5.2.  Semantics   The Auto-Submitted header field SHOULD NOT be supplied for messages   that were manually submitted by a human.  (However, user agents that   allow senders to specify arbitrary fields SHOULD NOT prevent humans   from setting the Auto-Submitted field, because it is sometimes useful   for testing.)   The auto-generated keyword:   -  SHOULD be used on messages generated by automatic (often periodic)      processes (such as UNIX "cron jobs") which are not direct      responses to other messages,   -  MUST NOT be used on manually generated messages,   -  MUST NOT be used on a message issued in direct response to another      message,   -  MUST NOT be used to label Delivery Status Notifications (DSNs)      [I2.RFC3464], or Message Disposition Notifications (MDNs)      [I3.RFC3798], or other reports of message (non)receipt or      (non)delivery.  Note: Some widely-deployed SMTP implementations      currently use "auto-generated" to label non-delivery reports.      These should be changed to use "auto-replied" instead.   The auto-replied keyword:   -  SHOULD be used on messages sent in direct response to another      message by an automatic process,Moore                       Standards Track                    [Page 14]

RFC 3834               Automatic E-Mail Responses            August 2004   -  MUST NOT be used on manually-generated messages,   -  MAY be used on Delivery Status Notifications (DSNs) and Message      Disposition Notifications (MDNs),   -  MUST NOT be used on messages generated by automatic or periodic      processes, except for messages which are automatic responses to      other messages.   The "no" keyword MAY be used to explicitly indicate that a message   was originated by a human, if for some reason this is found to be   appropriate.   Extension keywords may be defined in the future, though it seems   unlikely.  The syntax and semantics of such keywords must be   published as RFCs and approved using the IETF Consensus process   [N8.RFC2434].  Keywords beginning with "x-" are reserved for   experiments and use among consenting parties.  Recipients of messages   containing an Auto-Submitted field with any keyword other than "no"   MAY assume that the message was not manually submitted by a human.   Optional parameters may also be defined by an IETF Consensus process.   The syntax of optional parameters is given here to allow for future   definition should they be needed.  Implementations of Auto-Submitted   conforming to this specification MUST NOT fail to recognize an Auto-   Submitted field and keyword that contains syntactically valid   optional parameters, but such implementations MAY ignore those   parameters if they are present.  Parameter names beginning with "x-"   are reserved for experiments and use among consenting parties.   The "comment" syntactical construct from [N2.RFC2822] can be used to   indicate a reason why this message was automatically submitted.6.  Security Considerations   Automatic responders introduce the potential for several kinds of   attack, including:   -  Use of such responders to relay harmful or abusive content (worms,      viruses, spam, and spymail) for the purpose of wider distribution      of the content or masking the source of such content;   -  Use of such responders to mount denial-of-service attacks by using      responders to relay messages to large numbers of addresses, or to      flood individual mailboxes with a large amount of unwanted      content, or both;Moore                       Standards Track                    [Page 15]

RFC 3834               Automatic E-Mail Responses            August 2004   -  Deliberate or accidental use of such responders to construct mail      loops or "sorcerer's apprentice mode", thus taxing the resources      of the mail transport system;   -  Use of such responders to determine whether recipient addresses      are valid, especially when such information is not otherwise      provided (e.g., SMTP RCPT or VRFY command responses) and is not      intended to be disclosed;   -  Use of such responders to obtain personal information about      recipients, including information about recipients' recent usage      of his mailbox or recent activity;   -  In addition, the responder itself may be subject to attack by      sending it large numbers of requests.   This document attempts to reduce the vulnerability of responders to   such attack, in particular by   -  Recommending that responders not relay significant content from      the subject message (thus minimizing the potential for use of      responders to launder or amplify attacker-chosen content)   -  Recommending that responders clearly mark responses with the      "Auto-Submitted: auto-replied" header field to distinguish them      from messages originated by humans (in part, to minimize the      potential for loops and denial-of-service attacks),   -  Recommending that Personal and Group Responders limit the number      of responses sent to any individual per period of time (also      limiting the potential damage caused by loops),   -  Recommending that responders respond to at most one address per      incoming message (to minimize the potential for deliberate or      accidental denial-of-service via "multiplication" or sorcerer's      apprentice mode),   -  Recommending that responses from Personal and Group Responders      should be brief and in plain text format (to minimize the      potential for mail responders to be used as mechanisms for      transmitting harmful content and/or disguising the source of      harmful content).   However, because email addresses are easily forged, attacks are still   possible for any email responder which does not limit access and   require authentication before issuing a response.  The above measures   attempt to limit the damage which can be done, but they cannot   entirely prevent attacks.Moore                       Standards Track                    [Page 16]

RFC 3834               Automatic E-Mail Responses            August 2004   This section describes vulnerabilities inherent in automatically   responding to mail.  Other vulnerabilities are associated with some   mail-based services which automatically respond to email messages,   but these are not caused by the fact that the server automatically   responds to incoming messages.  In general, any network-based service   (including those accessed by email) needs to provide security that is   sufficient to prevent the service from being used as a means to   inappropriately or destructively access the resources that are   accessible by the service.   It has also been noted that Personal and Group Responders sometimes   inappropriately disclose recipients' personal information.  This   might happen automatically (as when a Group Responder automatically   supplies a recipient's personal or mobile telephone number as   alternate contact information) or "manually".  Automatically-   generated information SHOULD NOT include personal information about   the recipient which is not already known to, or easily available to,   the sender of the subject message.  User interfaces which allow   recipients to supply response text SHOULD make it clear to the user   that this information will be made available not only to local   colleagues but also to the entire Internet, including potential   attackers.7.  Example: vacation program   This section illustrates how these recommendations might apply to a   hypothetical "vacation" program that had the purpose of responding to   a single recipient's mail during periods in which that recipient was   busy or absent and unable to respond personally.  This is intended as   illustration only and is not a normative part of this standard.   The vacation program is a Personal Responder.   The vacation program refuses to respond to any message which:   -  appears to be spam (for instance, if it has been labelled as      advertising by the sender or as potential spam by some      intermediary),   -  appears to contain a virus (for instance, if it contains an      executable attachment),   -  contains an Auto-Submitted header field,   -  has been sent a response within the previous 7 days,   -  does not contain one of the recipient's addresses in a To, CC,      Bcc, Resent-To, Resent-CC, or Resent-Bcc field,Moore                       Standards Track                    [Page 17]

RFC 3834               Automatic E-Mail Responses            August 2004   -  contains a Precedence field with a value of "list", "junk", or      "bulk",   -  does not have a Return-Path address, or   -  has a Return-Path address of <>, or a Return-Path address of a      form that is frequently used by non-delivery reports.   The format of the vacation response is as follows:   -  The From header field is set to a name and email address specified      by the user on whose behalf the responses are being sent.  (On      some systems it may be reasonable to have a default setting for      the From field of vacation responses that is based on the user's      account name and the domain name of the system.)   -  The Reply-To field is set only if explicitly configured by the      user on whose behalf the responses are being sent.  For example, a      user might direct replies to a secretary or co-worker who has been      delegated to handle important matters during his absence.   -  The To field contains the address of the recipient of the      response, as obtained from the Return-Path field of the subject      message.   -  The Date field contains the date and time at which the response      was generated.   -  The Subject field contains Auto: followed by a string chosen by      the user on whose behalf the responses are being sent.  A default      setting of something like "away from my mail" might be      appropriate.  If the Subject field contains non-ASCII characters      these are encoded per [N3.RFC2047].   -  The In-Reply-To and References fields are generated from the      subject message per [N2.RFC2822].   -  The Auto-Submitted field has the value "auto-replied".   -  The message body contains some text specified by the user on whose      behalf the response is being sent.  A brief summary of the subject      message is also included, consisting of From, To, Subject, Date,      and a few lines of message text from the subject message.  No      attachments or non-text bodyparts are included in the response.Moore                       Standards Track                    [Page 18]

RFC 3834               Automatic E-Mail Responses            August 2004   The SMTP MAIL FROM address of the message envelope is <>.  The RCPT   TO address in the message envelope is the address of the user to whom   the response is being sent.  NOTIFY=NEVER is also set in the RCPT TO   line if permitted by the SMTP server.8.  IANA ConsiderationsSection 5 of this document defines two new extension mechanisms - new   keywords for the Auto-Submitted header field, and new optional   parameters for the Auto-Submitted field.  If at any point in the   future new keywords or parameters are approved (through an IETF   Consensus process) it may be appropriate for IANA to create a   registry of such keywords or parameters.9.  Acknowledgments   In the mid-1990s Jeroen Houttuin of TERENA authored a series of   internet-drafts on "Behavior of Mail Based Servers", and in   particular, one document on "Answering Servers".  While these   documents were (to this author's knowledge) never formally published,   they provided the first well-reasoned argument (known to this author)   as to the best way for such servers to interface with email systems   and protocols.   The idea for the Auto-Submitted field comes from the X.400/MHS mail   system [I7.X420].  [I8.RFC2156] defined an "Autosubmitted" field for   use when gatewaying between X.400 and Internet mail.  Jacob Palme   wrote an internet-draft defining use of the "Auto-Submitted" field   for Internet mail, which made it through Last Call without   significant objections, but got stalled in an attempt to resolve   non-substantial objections.  The definition of Auto-Submitted in this   document is derived (i.e., slightly simplified) from the one in that   document, with some text stolen outright.   Thanks are also due to those who contributed suggestions to this   document: Russ Allbery, Adam Costello, Ned Freed, Lawrence   Greenfield, Arnt Gulbrandsen, Eric Hall, Tony Hansen, Vivek Khera,   Dan Kohn, Bruce Lilly, Charles Lindsey, der Mouse, Lyndon Nerenberg,   Richard Rognlie, Markus Stumpf, Florian Weimer, and Dan Wing.10.  References10.1.  Normative References   [N1.RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate                 Requirement Levels",BCP 14,RFC 2119, March 1997.Moore                       Standards Track                    [Page 19]

RFC 3834               Automatic E-Mail Responses            August 2004   [N2.RFC2822]  Resnick, P., Ed., "Internet Message Format",RFC 2822,                 April 2001.   [N3.RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail                 Extensions) Part Three: Message Header Extensions for                 Non-ASCII Text",RFC 2047, November 1996.   [N4.RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and                 Encoded Word Extensions: Character Sets, Languages, and                 Continuations",RFC 2231, November 1997.   [N5.RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP)                 Service Extension for Delivery Status Notifications                 (DSNs)",RFC 3461, January 2003.   [N6.RFC2234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for                 Syntax Specifications: ABNF",RFC 2234, November 1997.   [N7.RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet                 Mail Extensions (MIME) Part One: Format of Internet                 Message Bodies",RFC 2045, November 1996.   [N8.RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing                 an IANA Considerations Section in RFCs",BCP 26,RFC2434, October 1998.10.2.  Informative References   [I1.JARGON]   "Sorcerer's apprentice mode", originally from the                 Jargon file once maintained at MIT-AI and SAIL; now                 collected at various places on the net.  See e.g.,http://www.jargon.net/   [I2.RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message                 Format for Delivery Status Notifications",RFC 3464,                 January 2003.   [I3.RFC3798]  Hansen, T. and G. Vaudreuil, Eds., "Message Disposition                 Notifications",RFC 3798, May 2004.   [I4.RFC2076]  Palme, J., "Common Internet Message Headers",RFC 2076,                 February 1997.   [I5.RFC2369]  Neufeld, G. and J. Baer, "The Use of URLs as Meta-                 Syntax for Core Mail List Commands and their Transport                 through Message Header Fields",RFC 2369, July 1998.Moore                       Standards Track                    [Page 20]

RFC 3834               Automatic E-Mail Responses            August 2004   [I6.RFC822]   Crocker, D., "Standard for the format of ARPA Internet                 text messages", STD 11,RFC 822, August 1982.   [I7.X420]     CCITT Recommendation X.420 (1992 E). Information                 technology - Message Handling Systems (MHS):                 Interpersonal messaging system, 1992.   [I8.RFC2156]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):                 Mapping between X.400 andRFC 822/MIME",RFC 2156,                 January 1998.Author's Address   Keith Moore   Innovative Computing Laboratory   University of Tennessee, Knoxville   1122 Volunteer Blvd, #203   Knoxville, TN 37996-3450   EMail: moore@cs.utk.eduMoore                       Standards Track                    [Page 21]

RFC 3834               Automatic E-Mail Responses            August 2004Full Copyright Statement   Copyright (C) The Internet Society (2004).  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 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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Moore                       Standards Track                    [Page 22]