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rfc:rfc3834

Network Working Group K. Moore Request for Comments: 3834 University of Tennessee Category: Standards Track August 2004

     Recommendations for Automatic Responses to Electronic Mail

Status 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:
  1. "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,
  1. "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),
  1. "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),
  1. 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),
  1. Information services similar to those mentioned above except that

they are intended to accept messages from other programs, and

Moore Standards Track [Page 2] RFC 3834 Automatic E-Mail Responses August 2004

  1. 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:
  1. "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.
  1. "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.
  1. "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 that

Moore 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:
  1. 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

  1. 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.
  1. 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.
  1. 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.
  1. 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

  1. 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.
  1. 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.
  1. 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 2004

3.1. 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:
  1. To provide an indication of the party or agent on whose behalf the

response was sent,

  1. To provide an address to which a recipient of an inappropriate

response can request that the situation be corrected, and

  1. To diminish the potential for mail loops.
 The following behavior is thus recommended:
  1. 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.
  1. 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.
  1. 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 possible

Moore 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 characters

Moore 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] section
 3.6.4.

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.  See
 section 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 fields

Moore 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 and

Moore 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 of

Moore 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:
  1. SHOULD be used on messages generated by automatic (often periodic)

processes (such as UNIX "cron jobs") which are not direct

    responses to other messages,
  1. MUST NOT be used on manually generated messages,
  1. MUST NOT be used on a message issued in direct response to another

message,

  1. 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:
  1. 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

  1. MUST NOT be used on manually-generated messages,
  1. MAY be used on Delivery Status Notifications (DSNs) and Message

Disposition Notifications (MDNs),

  1. 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:
  1. 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;
  1. 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

  1. 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;
  1. 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;
  1. Use of such responders to obtain personal information about

recipients, including information about recipients' recent usage

    of his mailbox or recent activity;
  1. 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
  1. 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)
  1. 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),
  1. 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),
  1. 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),
  1. 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:
  1. appears to be spam (for instance, if it has been labelled as

advertising by the sender or as potential spam by some

    intermediary),
  1. appears to contain a virus (for instance, if it contains an

executable attachment),

  1. contains an Auto-Submitted header field,
  1. has been sent a response within the previous 7 days,
  1. 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

  1. contains a Precedence field with a value of "list", "junk", or

"bulk",

  1. does not have a Return-Path address, or
  1. 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:
  1. 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.)
  1. 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.
  1. The To field contains the address of the recipient of the

response, as obtained from the Return-Path field of the subject

    message.
  1. The Date field contains the date and time at which the response

was generated.

  1. 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].
  1. The In-Reply-To and References fields are generated from the

subject message per [N2.RFC2822].

  1. The Auto-Submitted field has the value "auto-replied".
  1. 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 Considerations

 Section 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. References

10.1. 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, RFC
               2434, 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 and RFC 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.edu

Moore Standards Track [Page 21] RFC 3834 Automatic E-Mail Responses August 2004

Full Copyright Statement

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 to the rights, licenses and restrictions contained in BCP 78, and
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Acknowledgement

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Moore Standards Track [Page 22]

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