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rfc:rfc3464

Network Working Group K. Moore Request for Comments: 3464 University of Tennessee Obsoletes: 1894 G. Vaudreuil Category: Standards Track Lucent Technologies

                                                          January 2003
  An Extensible Message Format for Delivery Status Notifications

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 (2003).  All Rights Reserved.

Abstract

 This memo defines a Multipurpose Internet Mail Extensions (MIME)
 content-type that may be used by a message transfer agent (MTA) or
 electronic mail gateway to report the result of an attempt to deliver
 a message to one or more recipients.  This content-type is intended
 as a machine-processable replacement for the various types of
 delivery status notifications currently used in Internet electronic
 mail.
 Because many messages are sent between the Internet and other
 messaging systems (such as X.400 or the so-called "Local Area Network
 (LAN)-based" systems), the Delivery Status Notification (DSN)
 protocol is designed to be useful in a multi-protocol messaging
 environment.  To this end, the protocol described in this memo
 provides for the carriage of "foreign" addresses and error codes, in
 addition to those normally used in Internet mail.  Additional
 attributes may also be defined to support "tunneling" of foreign
 notifications through Internet mail.

Moore & Vaudreuil Standards Track [Page 1] RFC 3464 Delivery Status Notifications January 2003

Table of Contents

 1. Introduction ....................................................3
   1.1 Purposes .....................................................3
   1.2 Requirements .................................................4
   1.3 Terminology ..................................................5
 2. Format of a Delivery Status Notification ........................7
   2.1 The message/delivery-status content-type .....................9
    2.1.1 General conventions for DSN fields ........................9
    2.1.2 "*-type" sub-fields .......................................9
    2.1.3 Lexical tokens imported from RFC 822 .....................10
   2.2 Per-Message DSN Fields ......................................11
    2.2.1 The Original-Envelope-Id field ...........................11
    2.2.2 The Reporting-MTA DSN field ..............................12
    2.2.3 The DSN-Gateway field ....................................13
    2.2.4 The Received-From-MTA DSN field ..........................14
    2.2.5 The Arrival-Date DSN field ...............................14
   2.3 Per-Recipient DSN fields ....................................14
    2.3.1 Original-Recipient field .................................15
    2.3.2 Final-Recipient field ....................................15
    2.3.3 Action field .............................................16
    2.3.4 Status field .............................................18
    2.3.5 Remote-MTA field .........................................19
    2.3.6 Diagnostic-Code field ....................................19
    2.3.7 Last-Attempt-Date field ..................................20
    2.3.8 final-log-id field .......................................20
    2.3.9 Will-Retry-Until field ...................................20
   2.4 Extension fields ............................................21
 3. Conformance and Usage Requirements .............................22
 4. Security Considerations ........................................23
   4.1 Forgery .....................................................23
   4.2 Confidentiality .............................................23
   4.3 Non-Repudiation .............................................25
 5. References .....................................................25
 6. Acknowledgments ................................................26
 Appendix A - Collected Grammar ....................................27
 Appendix B - Guidelines for Gatewaying DSNS .......................29
   Gatewaying from other mail systems to DSNs ......................29
   Gatewaying from DSNs to other mail systems ......................30
 Appendix C - Guidelines for Use of DSNS By Mailing List Exploders .30
 Appendix D - IANA Registration Forms for DSN Types ................31
   IANA registration form for address-type .........................32
   IANA registration form for diagnostic-type ......................32
   IANA registration form for MTA-name-type ........................32
 Appendix E - Examples .............................................33
   Simple DSN ......................................................34
   Multi-Recipient DSN .............................................35
   DSN from gateway to foreign system ..............................36

Moore & Vaudreuil Standards Track [Page 2] RFC 3464 Delivery Status Notifications January 2003

   Delayed DSN .....................................................37
 Appendix F - Changes from RFC 1894 ................................38
 Authors' Addresses ................................................39
 Full Copyright Statement ..........................................40

1. Introduction

 This memo defines a Multipurpose Internet Mail Extensions (MIME)
 [MIME1] content-type for Delivery Status Notifications (DSNs).  A DSN
 can be used to notify the sender of a message of any of several
 conditions: failed delivery, delayed delivery, successful delivery,
 or the gatewaying of a message into an environment that may not
 support DSNs.  The "message/delivery-status" content-type defined
 herein is intended for use within the framework of the
 "multipart/report" content type defined in [REPORT].
 This memo defines only the format of the notifications.  An extension
 to the Simple Message Transfer Protocol (SMTP) [SMTP] to fully
 support such notifications is the subject of a separate memo [DRPT].
 Document Conventions
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119
 [RFC2119].

1.1 Purposes

 The DSNs defined in this memo are expected to serve several purposes:
 (a) Inform human beings of the status of message delivery processing,
     as well as the reasons for any delivery problems or outright
     failures, in a manner that is largely independent of human
     language and media;
 (b) Allow mail user agents to keep track of the delivery status of
     messages sent, by associating returned DSNs with earlier message
     transmissions;
 (c) Allow mailing list exploders to automatically maintain their
     subscriber lists when delivery attempts repeatedly fail;
 (d) Convey delivery and non-delivery notifications resulting from
     attempts to deliver messages to "foreign" mail systems via a
     gateway;

Moore & Vaudreuil Standards Track [Page 3] RFC 3464 Delivery Status Notifications January 2003

 (e) Allow "foreign" notifications to be tunneled through a MIME-
     capable message system and back into the original messaging
     system that issued the original notification, or even to a third
     messaging system;
 (f) Allow language-independent and medium-independent, yet reasonably
     precise, indications of the reason for the failure of a message
     to be delivered; and
 (g) Provide sufficient information to remote MTA maintainers (via
     "trouble tickets") so that they can understand the nature of
     reported errors.  This feature is used in the case that failure
     to deliver a message is due to the malfunction of a remote MTA
     and the sender wants to report the problem to the remote MTA
     administrator.

1.2 Requirements

 These purposes place the following constraints on the notification
 protocol:
 (a) It must be readable by humans as well as being machine-parsable.
 (b) It must provide enough information to allow message senders (or
     the user agents) to unambiguously associate a DSN with the
     message that was sent and the original recipient address for
     which the DSN is issued (if such information is available), even
     if the message was forwarded to another recipient address.
 (c) It must be able to preserve the reason for the success or failure
     of a delivery attempt in a remote messaging system, using the
     "language" (mailbox addresses and status codes) of that remote
     system.
 (d) It must also be able to describe the reason for the success or
     failure of a delivery attempt, independent of any particular
     human language or of the "language" of any particular mail
     system.
 (e) It must preserve enough information to allow the maintainer of a
     remote MTA to understand (and if possible, reproduce) the
     conditions that caused a delivery failure at that MTA.
 (f) For any notifications issued by foreign mail systems, which are
     translated by a mail gateway to the DSN format, the DSN must
     preserve the "type" of the foreign addresses and error codes, so
     that these may be correctly interpreted by gateways.

Moore & Vaudreuil Standards Track [Page 4] RFC 3464 Delivery Status Notifications January 2003

 A DSN contains a set of per-message fields that identify the message
 and the transaction during which the message was submitted, along
 with other fields that apply to all delivery attempts described by
 the DSN.  The DSN also includes a set of per-recipient fields to
 convey the result of the attempt to deliver the message to each of
 one or more recipients.

1.3 Terminology

 A message may be transmitted through several message transfer agents
 (MTAs) on its way to a recipient.  For a variety of reasons,
 recipient addresses may be rewritten during this process, so each MTA
 may potentially see a different recipient address.  Depending on the
 purpose for which a DSN is used, different formats of a particular
 recipient address will be needed.
 Several DSN fields are defined in terms of the view from a particular
 MTA in the transmission.  The MTAs are assigned the following names:
 (a) Original MTA
     The Original MTA is the one to which the message is submitted for
     delivery by the sender of the message.
 (b) Reporting MTA
     For any DSN, the Reporting MTA is the one which is reporting the
     results of delivery attempts described in the DSN.
     If the delivery attempts described occurred in a "foreign" (non-
     Internet) mail system, and the DSN was produced by translating
     the foreign notice into DSN format, the Reporting MTA will still
     identify the "foreign" MTA where the delivery attempts occurred.
 (c) Received-From MTA
     The Received-From MTA is the MTA from which the Reporting MTA
     received the message, and accepted responsibility for delivery of
     the message.
 (d) Remote MTA
     If an MTA determines that it must relay a message to one or more
     recipients, but the message cannot be transferred to its "next
     hop" MTA, or if the "next hop" MTA refuses to accept
     responsibility for delivery of the message to one or more of its
     intended recipients, the relaying MTA may need to issue a DSN on
     behalf of the recipients for whom the message cannot be

Moore & Vaudreuil Standards Track [Page 5] RFC 3464 Delivery Status Notifications January 2003

     delivered.  In this case the relaying MTA is the Reporting MTA,
     and the "next hop" MTA is known as the Remote MTA.
 Figure 1 illustrates the relationship between the various MTAs.

+—–+ +——–+ +———+ +———+ +——+

Received-
Original ⇒ … ⇒ From Reporting ==⇒ Remote
user MTA MTA MTA <No! MTA
agent

+—–+ (DSN returned to sender by Reporting MTA)

   Figure 1. Original, Received-From, Reporting and Remote MTAs
 Each of these MTAs may provide information that is useful in a DSN:
 + Ideally, the DSN will contain the address of each recipient as
   originally specified to the Original MTA by the sender of the
   message.
   This version of the address is needed (rather than a forwarding
   address or some modified version of the original address) so that
   the sender may compare the recipient address in the DSN with the
   address in the sender's records (e.g., an address book for an
   individual, the list of subscribers for a mailing list) and take
   appropriate action.
   Similarly, the DSN might contain an "envelope identifier" that was
   known to both the sender's user agent and the Original MTA at the
   time of message submission, and which, if included in the DSN, can
   be used by the sender to keep track of which messages were or were
   not delivered.
 + If a message was (a) forwarded to a different address than that
   specified by the sender, (b) gatewayed to a different mail system
   than that used by the sender, or (c) subjected to address rewriting
   during transmission, the "final" form of the recipient address
   (i.e., the one seen by the Reporting MTA) will be different than
   the original (sender-specified) recipient address.  Just as the
   sender's user agent (or the sender) prefers the original recipient
   address, so the "final" address is needed when reporting a problem
   to the postmaster of the site where message delivery failed,
   because only the final recipient address will allow her to
   reproduce the conditions that caused the failure.

Moore & Vaudreuil Standards Track [Page 6] RFC 3464 Delivery Status Notifications January 2003

 + A "failed" DSN should contain the most accurate explanation for the
   delivery failure that is available.  For ease of interpretation,
   this information should be a format that is independent of the mail
   transport system that issued the DSN.  However, if a foreign error
   code is translated into some transport-independent format, some
   information may be lost.  It is therefore desirable to provide both
   a transport-independent status code and a mechanism for reporting
   transport-specific codes.  Depending on the circumstances that
   produced delivery failure, the transport-specific code might be
   obtained from either the Reporting MTA or the Remote MTA.
 Since different values for "recipient address" and "delivery status
 code" are needed according to the circumstance in which a DSN will be
 used, and since the MTA that issues the DSN cannot anticipate those
 circumstances, the DSN format described here may contain both the
 original and final forms of a recipient address, and both a
 transport-independent and a transport-specific indication of delivery
 status.
 Extension fields may also be added by the Reporting MTA as needed to
 provide additional information for use in a trouble ticket or to
 preserve information for tunneling of foreign delivery reports
 through Internet DSNs.
 The Original, Reporting, and Remote MTAs may exist in very different
 environments and use dissimilar transport protocols, MTA names,
 address formats, and delivery status codes.  DSNs therefore do not
 assume any particular format for mailbox addresses, MTA names, or
 transport-specific status codes.  Instead, the various DSN fields
 that carry such quantities consist of a "type" sub-field followed by
 a sub-field whose contents are ordinary text characters, and the
 format of which is indicated by the "type" sub-field.  This allows a
 DSN to convey these quantities regardless of format.

2. Format of a Delivery Status Notification

 A DSN is a MIME message with a top-level content-type of
 multipart/report (defined in [REPORT]).  When a multipart/report
 content is used to transmit a DSN:
 (a) The report-type parameter of the multipart/report content is
     "delivery-status".
 (b) The first component of the multipart/report contains a human-
     readable explanation of the DSN, as described in [REPORT].

Moore & Vaudreuil Standards Track [Page 7] RFC 3464 Delivery Status Notifications January 2003

 (c) The second component of the multipart/report is of content-type
     message/delivery-status, described in section 2.1 of this
     document.
 (d) If the original message or a portion of the message is to be
     returned to the sender, it appears as the third component of the
     multipart/report.
 NOTE: For delivery status notifications gatewayed from foreign
 systems, the headers of the original message may not be available.
 In this case the third component of the DSN may be omitted, or it may
 contain "simulated" RFC 822 headers that contain equivalent
 information.  In particular, it is very desirable to preserve the
 subject, date, and message-id (or equivalent) fields from the
 original message.
 The DSN MUST be addressed (in both the message header and the
 transport envelope) to the return address from the transport envelope
 which accompanied the original message for which the DSN was
 generated.  (For a message that arrived via SMTP, the envelope return
 address appears in the MAIL FROM command.)
 The From field of the message header of the DSN SHOULD contain the
 address of a human who is responsible for maintaining the mail system
 at the Reporting MTA site (e.g., Postmaster), so that a reply to the
 DSN will reach that person.  Exception: if a DSN is translated from a
 foreign delivery report, and the gateway performing the translation
 cannot determine the appropriate address, the From field of the DSN
 MAY be the address of a human who is responsible for maintaining the
 gateway.
 The envelope sender address of the DSN SHOULD be chosen to ensure
 that no delivery status reports will be issued in response to the DSN
 itself, and MUST be chosen so that DSNs will not generate mail loops.
 Whenever an SMTP transaction is used to send a DSN, the MAIL FROM
 command MUST use a NULL return address, i.e., "MAIL FROM:<>".
 A particular DSN describes the delivery status for exactly one
 message.  However, an MTA MAY report on the delivery status for
 several recipients of the same message in a single DSN.  Due to the
 nature of the mail transport system (where responsibility for
 delivery of a message to its recipients may be split among several
 MTAs, and delivery to any particular recipient may be delayed),
 multiple DSNs may still be issued in response to a single message
 submission.

Moore & Vaudreuil Standards Track [Page 8] RFC 3464 Delivery Status Notifications January 2003

2.1 The message/delivery-status content-type

 The message/delivery-status content-type is defined as follows:
 MIME type name:             message
 MIME subtype name:          delivery-status
 Optional parameters:        none
 Encoding considerations:    "7bit" encoding is sufficient and
                             MUST be used to maintain readability
                             when viewed by non-MIME mail readers.
 Security considerations:    discussed in section 4 of this memo.
 The message/delivery-status report type for use in the
 multipart/report is "delivery-status".
 The body of a message/delivery-status consists of one or more
 "fields" formatted according to the ABNF of RFC 822 header "fields"
 (see [RFC822]).  The per-message fields appear first, followed by a
 blank line.  Following the per-message fields are one or more groups
 of per-recipient fields.  Each group of per-recipient fields is
 preceded by a blank line.  Using the ABNF of RFC 822, the syntax of
 the message/delivery-status content is as follows:
         delivery-status-content =  per-message-fields 1*
                                   ( CRLF per-recipient-fields )
 The per-message fields are described in section 2.2.  The
 per-recipient fields are described in section 2.3.

2.1.1 General conventions for DSN fields

 Since these fields are defined according to the rules of RFC 822, the
 same conventions for continuation lines and comments apply.
 Notification fields may be continued onto multiple lines by beginning
 each additional line with a SPACE or HTAB.  Text that appears in
 parentheses is considered a comment and not part of the contents of
 that notification field.  Field names are case-insensitive, so the
 names of notification fields may be spelled in any combination of
 upper and lower case letters.  Comments in DSN fields may use the
 "encoded-word" construct defined in [MIME3].

2.1.2 "*-type" sub-fields

 Several DSN fields consist of a "-type" sub-field, followed by a
 semicolon, followed by "*text".  For these fields, the keyword used
 in the address-type, diagnostic-type, or MTA-name-type sub-field
 indicates the expected format of the address, status-code, or
 MTA-name which follows.

Moore & Vaudreuil Standards Track [Page 9] RFC 3464 Delivery Status Notifications January 2003

 The "-type" sub-fields are defined as follows:
 (a) An "address-type" specifies the format of a mailbox address.  For
     example, Internet mail addresses use the "rfc822" address-type.
             address-type = atom
 (b) A "diagnostic-type" specifies the format of a status code.  For
     example, when a DSN field contains a reply code reported via the
     Simple Mail Transfer Protocol [SMTP], the "smtp" diagnostic-type
     is used.
             diagnostic-type = atom
 (c) An "MTA-name-type" specifies the format of an MTA name.  For
     example, for an SMTP server on an Internet host, the MTA name is
     the domain name of that host, and the "dns" MTA-name-type is
     used.
             mta-name-type = atom
 Values for address-type, diagnostic-type, and MTA-name-type are
 case-insensitive.  Thus address-type values of "RFC822" and "rfc822"
 are equivalent.
 The Internet Assigned Numbers Authority (IANA) will maintain a
 registry of address-types, diagnostic-types, and MTA-name-types,
 along with descriptions of the meanings and acceptable values of
 each, or a reference to one or more specifications that provide such
 descriptions.  (The "rfc822" address-type, "smtp" diagnostic-type,
 and "dns" MTA-name-type are defined in [DRPT].)  Registration forms
 for address-type, diagnostic-type, and MTA-name-type appear in
 Appendix D.
 IANA will not accept registrations for any address-type,
 diagnostic-type, or MTA-name-type name that begins with "X-".  These
 type names are reserved for experimental use.

2.1.3 Lexical tokens imported from RFC 822

 The following lexical tokens, defined in [RFC822], are used in the
 ABNF grammar for DSNs: atom, CHAR, comment, CR, CRLF, DIGIT, LF,
 linear-white-space, SPACE, text.  The date-time lexical token is
 defined in [HOSTREQ].

Moore & Vaudreuil Standards Track [Page 10] RFC 3464 Delivery Status Notifications January 2003

2.2 Per-Message DSN Fields

 Some fields of a DSN apply to all of the delivery attempts described
 by that DSN.  At most, these fields may appear once in any DSN.
 These fields are used to correlate the DSN with the original message
 transaction and to provide additional information which may be useful
 to gateways.
        per-message-fields =
              [ original-envelope-id-field CRLF ]
              reporting-mta-field CRLF
              [ dsn-gateway-field CRLF ]
              [ received-from-mta-field CRLF ]
              [ arrival-date-field CRLF ]
              *( extension-field CRLF )

2.2.1 The Original-Envelope-Id field

 The optional Original-Envelope-Id field contains an "envelope
 identifier" that uniquely identifies the transaction during which the
 message was submitted, and was either (a) specified by the sender and
 supplied to the sender's MTA, or (b) generated by the sender's MTA
 and made available to the sender when the message was submitted.  Its
 purpose is to allow the sender (or her user agent) to associate the
 returned DSN with the specific transaction in which the message was
 sent.
 If such an envelope identifier was present in the envelope that
 accompanied the message when it arrived at the Reporting MTA, it
 SHOULD be supplied in the Original-Envelope-Id field of any DSNs
 issued as a result of an attempt to deliver the message.  Except when
 a DSN is issued by the sender's MTA, an MTA MUST NOT supply this
 field unless there is an envelope-identifier field in the envelope
 that accompanied this message on its arrival at the Reporting MTA.
 The Original-Envelope-Id field is defined as follows:
         original-envelope-id-field =
                "Original-Envelope-Id" ":" envelope-id
          envelope-id = *text
 There may be at most one Original-Envelope-Id field per DSN.
 The envelope-id is CASE-SENSITIVE.  The DSN MUST preserve the
 original case and spelling of the envelope-id.

Moore & Vaudreuil Standards Track [Page 11] RFC 3464 Delivery Status Notifications January 2003

       NOTE: The Original-Envelope-Id is NOT the same as the
       Message-Id from the message header.  The Message-Id identifies
       the content of the message, while the Original-Envelope-Id
       identifies the transaction in which the message is sent.

2.2.2 The Reporting-MTA DSN field

       reporting-mta-field =
             "Reporting-MTA" ":" mta-name-type ";" mta-name
        mta-name = *text
 The Reporting-MTA field is defined as follows:
 A DSN describes the results of attempts to deliver, relay, or gateway
 a message to one or more recipients.  In all cases, the Reporting-MTA
 is the MTA that attempted to perform the delivery, relay, or gateway
 operation described in the DSN.  This field is required.
 Note that if an SMTP client attempts to relay a message to an SMTP
 server and receives an error reply to a RCPT command, the client is
 responsible for generating the DSN, and the client's domain name will
 appear in the Reporting-MTA field.  (The server's domain name will
 appear in the Remote-MTA field.)
 Note that the Reporting-MTA is not necessarily the MTA which actually
 issued the DSN.  For example, if an attempt to deliver a message
 outside of the Internet resulted in a non-delivery notification which
 was gatewayed back into Internet mail, the Reporting-MTA field of the
 resulting DSN would be that of the MTA that originally reported the
 delivery failure, not that of the gateway which converted the foreign
 notification into a DSN.  See Figure 2.

Moore & Vaudreuil Standards Track [Page 12] RFC 3464 Delivery Status Notifications January 2003

sender's environment recipient's environment ………………………. ……………………………………

                          : :
                      (1) : :                             (2)
 +-----+  +--------+  +--------+  +---------+  +---------+   +------+
 |     |  |        |  |        |  |Received-|  |         |   |      |
 |     |=>|Original|=>|        |->|  From   |->|Reporting|-->|Remote|
 | user|  |   MTA  |  |        |  |   MTA   |  |   MTA   |<No|  MTA |
 |agent|  +--------+  |Gateway |  +---------+  +----v----+   +------+
 |     |              |        |                    |
 |     | <============|        |<-------------------+
 +-----+              |        |(4)                (3)
                      +--------+
                          : :

………………………: :…………………………………..

            Figure 2. DSNs in the presence of gateways
 (1) message is gatewayed into recipient's environment
 (2) attempt to relay message fails
 (3) reporting-mta (in recipient's environment) returns non-delivery
     notification
 (4) gateway translates foreign notification into a DSN
 The mta-name portion of the Reporting-MTA field is formatted
 according to the conventions indicated by the mta-name-type
 sub-field.  If an MTA functions as a gateway between dissimilar mail
 environments and thus is known by multiple names depending on the
 environment, the mta-name sub-field SHOULD contain the name used by
 the environment from which the message was accepted by the
 Reporting-MTA.
 Because the exact spelling of an MTA name may be significant in a
 particular environment, MTA names are CASE-SENSITIVE.

2.2.3 The DSN-Gateway field

 The DSN-Gateway field indicates the name of the gateway or MTA that
 translated a foreign (non-Internet) delivery status notification into
 this DSN.  This field MUST appear in any DSN that was translated by a
 gateway from a foreign system into DSN format, and MUST NOT appear
 otherwise.
    dsn-gateway-field = "DSN-Gateway" ":" mta-name-type ";" mta-name
 For gateways into Internet mail, the MTA-name-type will normally be
 "dns", and the mta-name will be the Internet domain name of the
 gateway.

Moore & Vaudreuil Standards Track [Page 13] RFC 3464 Delivery Status Notifications January 2003

2.2.4 The Received-From-MTA DSN field

 The optional Received-From-MTA field indicates the name of the MTA
 from which the message was received.
      received-from-mta-field =
           "Received-From-MTA" ":" mta-name-type ";" mta-name
 If the message was received from an Internet host via SMTP, the
 contents of the mta-name sub-field SHOULD be the Internet domain name
 supplied in the HELO or EHLO command, and the network address used by
 the SMTP client SHOULD be included as a comment enclosed in
 parentheses.  (In this case, the MTA-name-type will be "dns".)
 The mta-name portion of the Received-From-MTA field is formatted
 according to the conventions indicated by the MTA-name-type sub-
 field.
 Since case is significant in some mail systems, the exact spelling,
 including case, of the MTA name SHOULD be preserved.

2.2.5 The Arrival-Date DSN field

 The optional Arrival-Date field indicates the date and time at which
 the message arrived at the Reporting MTA.  If the Last-Attempt-Date
 field is also provided in a per-recipient field, this can be used to
 determine the interval between when the message arrived at the
 Reporting MTA and when the report was issued for that recipient.
      arrival-date-field = "Arrival-Date" ":" date-time
 The date and time are expressed in RFC 822 'date-time' format, as
 modified by [HOSTREQ].  Numeric timezones ([+/-]HHMM format) MUST be
 used.

2.3 Per-Recipient DSN fields

 A DSN contains information about attempts to deliver a message to one
 or more recipients.  The delivery information for any particular
 recipient is contained in a group of contiguous per-recipient fields.
 Each group of per-recipient fields is preceded by a blank line.

Moore & Vaudreuil Standards Track [Page 14] RFC 3464 Delivery Status Notifications January 2003

 The syntax for the group of per-recipient fields is as follows:
      per-recipient-fields =
            [ original-recipient-field CRLF ]
            final-recipient-field CRLF
            action-field CRLF
            status-field CRLF
            [ remote-mta-field CRLF ]
            [ diagnostic-code-field CRLF ]
            [ last-attempt-date-field CRLF ]
            [ final-log-id-field CRLF ]
            [ will-retry-until-field CRLF ]
            *( extension-field CRLF )

2.3.1 Original-Recipient field

 The Original-Recipient field indicates the original recipient address
 as specified by the sender of the message for which the DSN is being
 issued.
     original-recipient-field =
           "Original-Recipient" ":" address-type ";" generic-address
      generic-address = *text
 The address-type field indicates the type of the original recipient
 address.  If the message originated within the Internet, the
 address-type field will normally be "rfc822", and the address will be
 according to the syntax specified in [RFC822].  The value "unknown"
 should be used if the Reporting MTA cannot determine the type of the
 original recipient address from the message envelope.
 This field is optional.  It should be included only if the sender-
 specified recipient address was present in the message envelope, such
 as by the SMTP extensions defined in [DRPT].  This address is the
 same as that provided by the sender and can be used to automatically
 correlate DSN reports and message transactions.

2.3.2 Final-Recipient field

 The Final-Recipient field indicates the recipient for which this set
 of per-recipient fields applies.  This field MUST be present in each
 set of per-recipient data.

Moore & Vaudreuil Standards Track [Page 15] RFC 3464 Delivery Status Notifications January 2003

 The syntax of the field is as follows:
       final-recipient-field =
           "Final-Recipient" ":" address-type ";" generic-address
 The generic-address sub-field of the Final-Recipient field MUST
 contain the mailbox address of the recipient (from the transport
 envelope), as it was when the Reporting MTA accepted the message for
 delivery.
 The Final-Recipient address may differ from the address originally
 provided by the sender, because it may have been transformed during
 forwarding and gatewaying into a totally unrecognizable mess.
 However, in the absence of the optional Original-Recipient field, the
 Final-Recipient field and any returned content may be the only
 information available with which to correlate the DSN with a
 particular message submission.
 The address-type sub-field indicates the type of address expected by
 the reporting MTA in that context.  Recipient addresses obtained via
 SMTP will normally be of address-type "rfc822".
 NOTE: The Reporting MTA is not expected to ensure that the address
 actually conforms to the syntax conventions of the address-type.
 Instead, it MUST report exactly the address received in the envelope,
 unless that address contains characters such as CR or LF which are
 not allowed in a DSN field.
 Since mailbox addresses (including those used in the Internet) may be
 case sensitive, the case of alphabetic characters in the address MUST
 be preserved.

2.3.3 Action field

 The Action field indicates the action performed by the Reporting-MTA
 as a result of its attempt to deliver the message to this recipient
 address.  This field MUST be present for each recipient named in the
 DSN.
 The syntax for the action-field is:
    action-field = "Action" ":" action-value
    action-value =
          "failed" / "delayed" / "delivered" / "relayed" / "expanded"
 The action-value may be spelled in any combination of upper and lower
 case characters.

Moore & Vaudreuil Standards Track [Page 16] RFC 3464 Delivery Status Notifications January 2003

 "failed"    indicates that the message could not be delivered to the
             recipient.  The Reporting MTA has abandoned any attempts
             to deliver the message to this recipient.  No further
             notifications should be expected.
 "delayed"   indicates that the Reporting MTA has so far been unable
             to deliver or relay the message, but it will continue to
             attempt to do so.  Additional notification messages may
             be issued as the message is further delayed or
             successfully delivered, or if delivery attempts are later
             abandoned.
 "delivered" indicates that the message was successfully delivered to
             the recipient address specified by the sender, which
             includes "delivery" to a mailing list exploder.  It does
             not indicate that the message has been read.  This is a
             terminal state and no further DSN for this recipient
             should be expected.
 "relayed"   indicates that the message has been relayed or gatewayed
             into an environment that does not accept responsibility
             for generating DSNs upon successful delivery.  This
             action-value SHOULD NOT be used unless the sender has
             requested notification of successful delivery for this
             recipient.
 "expanded"  indicates that the message has been successfully
             delivered to the recipient address as specified by the
             sender, and forwarded by the Reporting-MTA beyond that
             destination to multiple additional recipient addresses.
             An action-value of "expanded" differs from "delivered" in
             that "expanded" is not a terminal state.  Further
             "failed" and/or "delayed" notifications may be provided.
 Using the terms "mailing list" and "alias" as defined in [DRPT],
 section 7.2.7: An action-value of "expanded" is only to be used when
 the message is delivered to a multiple-recipient "alias".  An
 action-value of "expanded" SHOULD NOT be used with a DSN issued on
 delivery of a message to a "mailing list".
     NOTE ON ACTION VS. STATUS CODES: Although the 'action' field
     might seem to be redundant with the 'status' field, this is not
     the case.  In particular, a "temporary failure" ("4") status code
     could be used with an action-value of either "delayed" or
     "failed".  For example, assume that an SMTP client repeatedly
     tries to relay a message to the mail exchanger for a recipient,
     but fails because a query to a domain name server timed out.

Moore & Vaudreuil Standards Track [Page 17] RFC 3464 Delivery Status Notifications January 2003

     After a few hours, it might issue a "delayed" DSN to inform the
     sender that the message had not yet been delivered.  After a few
     days, the MTA might abandon its attempt to deliver the message
     and return a "failed" DSN.  The status code (which would begin
     with a "4" to indicate "temporary failure") would be the same for
     both DSNs.
     Another example for which the action and status codes may appear
     contradictory: If an MTA or mail gateway cannot deliver a message
     because doing so would entail conversions resulting in an
     unacceptable loss of information, it would issue a DSN with the
     'action' field of "failure" and a status code of 'XXX'.  If the
     message had instead been relayed, but with some loss of
     information, it might generate a DSN with the same XXX status-
     code, but with an action field of "relayed".

2.3.4 Status field

 The per-recipient Status field contains a transport-independent
 status code that indicates the delivery status of the message to that
 recipient.  This field MUST be present for each delivery attempt
 which is described by a DSN.
 The syntax of the status field is:
 status-field = "Status" ":" status-code
 status-code = DIGIT "." 1*3DIGIT "." 1*3DIGIT
    ; White-space characters and comments are NOT allowed within
    ; a status-code, though a comment enclosed in parentheses
    ; MAY follow the last numeric sub-field of the status-code.
    ; Each numeric sub-field within the status-code MUST be
    ; expressed without leading zero digits.
 Status codes thus consist of three numerical fields separated by ".".
 The first sub-field indicates whether the delivery attempt was
 successful (2= success, 4 = persistent temporary failure, 5 =
 permanent failure).  The second sub-field indicates the probable
 source of any delivery anomalies, and the third sub-field denotes a
 precise error condition, if known.
 The initial set of status-codes is defined in [STATUS].

Moore & Vaudreuil Standards Track [Page 18] RFC 3464 Delivery Status Notifications January 2003

2.3.5 Remote-MTA field

 The value associated with the Remote-MTA DSN field is a printable
 ASCII representation of the name of the "remote" MTA that reported
 delivery status to the "reporting" MTA.
    remote-mta-field = "Remote-MTA" ":" mta-name-type ";" mta-name
 NOTE: The Remote-MTA field preserves the "while talking to"
 information that was provided in some pre-existing nondelivery
 reports.
 This field is optional.  It MUST NOT be included if no remote MTA was
 involved in the attempted delivery of the message to that recipient.

2.3.6 Diagnostic-Code field

 For a "failed" or "delayed" recipient, the Diagnostic-Code DSN field
 contains the actual diagnostic code issued by the mail transport.
 Since such codes vary from one mail transport to another, the
 diagnostic-type sub-field is needed to specify which type of
 diagnostic code is represented.
  diagnostic-code-field =
        "Diagnostic-Code" ":" diagnostic-type ";" *text
 NOTE: The information in the Diagnostic-Code field may be somewhat
 redundant with that from the Status field.  The Status field is
 needed so that any DSN, regardless of origin, may be understood by
 any user agent or gateway that parses DSNs.  Since the Status code
 will sometimes be less precise than the actual transport diagnostic
 code, the Diagnostic-Code field is provided to retain the latter
 information.  Such information may be useful in a trouble ticket sent
 to the administrator of the Reporting MTA, or when tunneling foreign
 non-delivery reports through DSNs.
 If the Diagnostic Code was obtained from a Remote MTA during an
 attempt to relay the message to that MTA, the Remote-MTA field should
 be present.  When interpreting a DSN, the presence of a Remote-MTA
 field indicates that the Diagnostic Code was issued by the Remote
 MTA.  The absence of a Remote-MTA indicates that the Diagnostic Code
 was issued by the Reporting MTA.
 In addition to the Diagnostic-Code itself, additional textual
 description of the diagnostic, MAY appear in a comment enclosed in
 parentheses.

Moore & Vaudreuil Standards Track [Page 19] RFC 3464 Delivery Status Notifications January 2003

 This field is optional, because some mail systems supply no
 additional information beyond that which is returned in the 'action'
 and 'status' fields.  However, this field SHOULD be included if
 transport-specific diagnostic information is available.

2.3.7 Last-Attempt-Date field

 The Last-Attempt-Date field gives the date and time of the last
 attempt to relay, gateway, or deliver the message (whether successful
 or unsuccessful) by the Reporting MTA.  This is not necessarily the
 same as the value of the Date field from the header of the message
 used to transmit this delivery status notification: In cases where
 the DSN was generated by a gateway, the Date field in the message
 header contains the time the DSN was sent by the gateway and the DSN
 Last-Attempt-Date field contains the time the last delivery attempt
 occurred.
    last-attempt-date-field = "Last-Attempt-Date" ":" date-time
 This field is optional.  It MUST NOT be included if the actual date
 and time of the last delivery attempt are not available (which might
 be the case if the DSN were being issued by a gateway).
 The date and time are expressed in RFC 822 'date-time' format, as
 modified by [HOSTREQ].  Numeric timezones ([+/-]HHMM format) MUST be
 used.

2.3.8 final-log-id field

 The "final-log-id" field gives the final-log-id of the message that
 was used by the final-mta.  This can be useful as an index to the
 final-mta's log entry for that delivery attempt.
    final-log-id-field = "Final-Log-ID" ":" *text
 This field is optional.

2.3.9 Will-Retry-Until field

 For DSNs of type "delayed", the Will-Retry-Until field gives the date
 after which the Reporting MTA expects to abandon all attempts to
 deliver the message to that recipient.  The Will-Retry-Until field is
 optional for "delay" DSNs, and MUST NOT appear in other DSNs.
    will-retry-until-field = "Will-Retry-Until" ":" date-time

Moore & Vaudreuil Standards Track [Page 20] RFC 3464 Delivery Status Notifications January 2003

 The date and time are expressed in RFC 822 'date-time' format, as
 modified by [HOSTREQ].  Numeric timezones ([+/-]HHMM format) MUST be
 used.

2.4 Extension fields

 Additional per-message or per-recipient DSN fields may be defined in
 the future by later revisions or extensions to this specification.
 Extension-field names beginning with "X-" will never be defined as
 standard fields;  such names are reserved for experimental use.  DSN
 field names NOT beginning with "X-" MUST be registered with the
 Internet Assigned Numbers Authority (IANA) and published in an RFC.
 Extension DSN fields may be defined for the following reasons:
 (a) To allow additional information from foreign delivery status
     reports to be tunneled through Internet DSNs.  The names of such
     DSN fields should begin with an indication of the foreign
     environment name (e.g., X400-Physical-Forwarding-Address).
 (b) To allow the transmission of diagnostic information which is
     specific to a particular mail transport protocol.  The names of
     such DSN fields should begin with an indication of the mail
     transport being used (e.g., SMTP-Remote-Recipient-Address).  Such
     fields should be used for diagnostic purposes only and not by
     user agents or mail gateways.
 (c) To allow transmission of diagnostic information which is specific
     to a particular message transfer agent (MTA).  The names of such
     DSN fields should begin with an indication of the MTA
     implementation that produced the DSN. (e.g., Foomail-Queue-ID).
 MTA implementers are encouraged to provide adequate information, via
 extension fields if necessary, to allow an MTA maintainer to
 understand the nature of correctable delivery failures and how to fix
 them.  For example, if message delivery attempts are logged, the DSN
 might include information that allows the MTA maintainer to easily
 find the log entry for a failed delivery attempt.
 If an MTA developer does not wish to register the meanings of such
 extension fields, "X-" fields may be used for this purpose.  To avoid
 name collisions, the name of the MTA implementation should follow the
 "X-", (e.g., "X-Foomail-Log-ID").

Moore & Vaudreuil Standards Track [Page 21] RFC 3464 Delivery Status Notifications January 2003

3. Conformance and Usage Requirements

 An MTA or gateway conforms to this specification if it generates DSNs
 according to the protocol defined in this memo.  For MTAs and
 gateways that do not support requests for positive delivery
 notification (such as in [DRPT]), it is sufficient that delivery
 failure reports use this protocol.
 A minimal implementation of this specification need generate only the
 Reporting-MTA per-message field, and the Final-Recipient, Action, and
 Status fields for each attempt to deliver a message to a recipient
 described by the DSN.  Generation of the other fields, when
 appropriate, is strongly recommended.
 MTAs and gateways MUST NOT generate the Original-Recipient field of a
 DSN unless the mail transfer protocol provides the address originally
 specified by the sender at the time of submission.  (Ordinary SMTP
 does not make that guarantee, but the SMTP extension defined in
 [DRPT] permits such information to be carried in the envelope if it
 is available.)
 Each sender-specified recipient address SHOULD result in at most one
 "delivered" or "failed" DSN for that recipient.  If a positive DSN is
 requested (e.g., one using NOTIFY=SUCCESS in SMTP) for a recipient
 that is forwarded to multiple recipients of an "alias" (as defined in
 [DRPT], section 7.2.7), the forwarding MTA SHOULD normally issue a
 "expanded" DSN for the originally-specified recipient and not
 propagate the request for a DSN to the forwarding addresses.
 Alternatively, the forwarding MTA MAY relay the request for a DSN to
 exactly one of the forwarding addresses and not propagate the request
 to the others.
 By contrast, successful submission of a message to a mailing list
 exploder is considered final delivery of the message.  Upon delivery
 of a message to a recipient address corresponding to a mailing list
 exploder, the Reporting MTA SHOULD issue an appropriate DSN exactly
 as if the recipient address were that of an ordinary mailbox.
     NOTE: This is actually intended to make DSNs usable by mailing
     lists themselves.  Any message sent to a mailing list subscriber
     should have its envelope return address pointing to the list
     maintainer [see RFC 1123, section 5.3.7(E)].  Since DSNs are sent
     to the envelope return address, all DSNs resulting from delivery
     to the recipients of a mailing list will be sent to the list
     maintainer.  The list maintainer may elect to mechanically
     process DSNs upon receipt, and thus automatically delete invalid
     addresses from the list. (See section 7 of this memo.)

Moore & Vaudreuil Standards Track [Page 22] RFC 3464 Delivery Status Notifications January 2003

 This specification places no restrictions on the processing of DSNs
 received by user agents or distribution lists.

4. Security Considerations

 The following security considerations apply when using DSNs:

4.1 Forgery

 DSNs may be forged as easily as ordinary Internet electronic mail.
 User agents and automatic mail handling facilities (such as mail
 distribution list exploders) that wish to make automatic use of DSNs
 should take appropriate precautions to minimize the potential damage
 from denial-of-service attacks.
 Security threats related to forged DSNs include the sending of:
 (a) A falsified delivery notification when the message is not
     delivered to the indicated recipient,
 (b) A falsified non-delivery notification when the message was in
     fact delivered to the indicated recipient,
 (c) A falsified Final-Recipient address,
 (d) A falsified Remote-MTA identification,
 (e) A falsified relay notification when the message is "dead ended".
 (f) Unsolicited DSNs

4.2 Confidentiality

 Another dimension of security is confidentiality.  There may be cases
 in which a message recipient is autoforwarding messages but does not
 wish to divulge the address to which the messages are autoforwarded.
 The desire for such confidentiality will probably be heightened as
 "wireless mailboxes", such as pagers, become more widely used as
 autoforward addresses.
 MTA authors are encouraged to provide a mechanism which enables the
 end user to preserve the confidentiality of a forwarding address.
 Depending on the degree of confidentiality required, and the nature
 of the environment to which a message were being forwarded, this
 might be accomplished by one or more of:

Moore & Vaudreuil Standards Track [Page 23] RFC 3464 Delivery Status Notifications January 2003

 (a) issuing a "relayed" DSN (if a positive DSN was requested) when a
     message is forwarded to a confidential forwarding address, and
     disabling requests for positive DSNs for the forwarded message,
 (b) declaring the message to be delivered, issuing a "delivered" DSN,
     re-sending the message to the confidential forwarding address,
     and arranging for no DSNs to be issued for the re-sent message,
 (c) omitting "Remote-*" or extension fields of a DSN whenever they
     would otherwise contain confidential information (such as a
     confidential forwarding address),
 (d) for messages forwarded to a confidential address, setting the
     envelope return address (e.g., SMTP MAIL FROM address) to the
     NULL reverse-path ("<>") (so that no DSNs would be sent from a
     downstream MTA to the original sender),
 (e) for messages forwarded to a confidential address, disabling
     delivery notifications for the forwarded message (e.g., if the
     "next-hop" MTA uses ESMTP and supports the DSN extension, by
     using the NOTIFY=NEVER parameter to the RCPT command), or
 (f) when forwarding mail to a confidential address, having the
     forwarding MTA rewrite the envelope return address for the
     forwarded message and attempt delivery of that message as if the
     forwarding MTA were the originator.  On its receipt of final
     delivery status, the forwarding MTA would issue a DSN to the
     original sender.
 In general, any optional DSN field may be omitted if the Reporting
 MTA site determines that inclusion of the field would impose too
 great a compromise of site confidentiality.  The need for such
 confidentiality must be balanced against the utility of the omitted
 information in trouble reports and DSNs gatewayed to foreign
 environments.
 Implementers are cautioned that many existing MTAs will send non-
 delivery notifications to a return address in the message header
 (rather than to the one in the envelope), in violation of SMTP and
 other protocols.  If a message is forwarded through such an MTA, no
 reasonable action on the part of the forwarding MTA will prevent the
 downstream MTA from compromising the forwarding address.  Likewise,
 if the recipient's MTA automatically responds to messages based on a
 request in the message header (such as the nonstandard, but widely
 used, Return-Receipt-To extension header), it will also compromise
 the forwarding address.

Moore & Vaudreuil Standards Track [Page 24] RFC 3464 Delivery Status Notifications January 2003

4.3 Non-Repudiation

 Within the framework of today's internet mail, the DSNs defined in
 this memo provide valuable information to the mail user; however,
 even a "failed" DSN can not be relied upon as a guarantee that a
 message was not received by the recipient.  Even if DSNs are not
 actively forged, conditions exist under which a message can be
 delivered despite the fact that a failure DSN was issued.
 For example, a race condition in the SMTP protocol allows for the
 duplication of messages if the connection is dropped following a
 completed DATA command, but before a response is seen by the SMTP
 client.
 This will cause the SMTP client to retransmit the message, even
 though the SMTP server has already accepted it [SMTPDUP].  If one of
 those delivery attempts succeeds and the other one fails, a "failed"
 DSN could be issued even though the message actually reached the
 recipient.

5. Normative References

 [DRPT]    Moore, K., "SMTP Service Extension for Delivery Status
           Notifications", RFC 3461, January 2003.
 [DSN]     Moore, K. and G. Vaudreuil, "An Extensible Message Format
           for Delivery Status Notifications", RFC 1894, January 1996.
 [HOSTREQ] Braden, R. (ed.), "Requirements for Internet Hosts -
           Application and Support", STD 3, RFC 1123, October 1989.
 [MIME1]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
           Extensions (MIME) Part One: Format of Internet Message
           Bodies", RFC 2045, November 1996.
 [MIME3]   Moore, K., "MIME (Multipurpose Internet Mail Extensions)
           Part Three: Message Header Extensions for Non-ASCII Text",
           RFC 2047, November 1996.
 [REPORT]  Vaudreuil, G., "The Multipart/Report Content Type for the
           Reporting of Mail System Administrative Messages", RFC
           3462, January 2003.
 [RFC822]  Crocker, D., "Standard for the format of ARPA Internet Text
           Messages", STD 11, RFC 822, August 1982.
 [SMTP]    Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
           821, August 1982.

Moore & Vaudreuil Standards Track [Page 25] RFC 3464 Delivery Status Notifications January 2003

 [SMTPDUP] Partridge, C., "Duplicate Messages and SMTP", RFC 1047,
           February 1988.
 [STATUS]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
           3463, January 2003.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

6. Acknowledgments

 The authors wish to thank the following people for their reviews of
 early drafts of RFC 1894, of which this document is a revision, and
 their suggestions for improvement:  Eric Allman, Harald Alvestrand,
 Allan Cargille, Jim Conklin, Peter Cowen, Dave Crocker, Roger Fajman,
 Ned Freed, Marko Kaittola, Steve Kille, John Klensin, John Gardiner
 Myers, Mark Nahabedian, Julian Onions, Jacob Palme, Jean Charles Roy,
 and Gregory Sheehan.

Moore & Vaudreuil Standards Track [Page 26] RFC 3464 Delivery Status Notifications January 2003

Appendix A - collected grammar

 NOTE: The following lexical tokens are defined in RFC 822: atom,
 CHAR, comment, CR, CRLF, DIGIT, LF, linear-white-space, SPACE, text.
 The date-time lexical token is defined in [HOSTREQ].
    action-field = "Action" ":" action-value
    action-value =  "failed" / "delayed" / "delivered"
          / "relayed" / "expanded"
    address-type = atom
    arrival-date-field = "Arrival-Date" ":" date-time
    delivery-status-content =  per-message-fields
          1*( CRLF per-recipient-fields )
    diagnostic-code-field =  "Diagnostic-Code" ":"
          diagnostic-type ";" *text
    diagnostic-type = atom
    dsn-gateway-field = "DSN-Gateway" ":" mta-name-type ";" mta-name
    envelope-id = *text
    extension-field = extension-field-name ":" *text
    extension-field-name = atom
    final-recipient-field =
          "Final-Recipient" ":" address-type ";" generic-address
    final-log-id-field = "Final-Log-ID" ":" *text
    generic-address = *text
    last-attempt-date-field = "Last-Attempt-Date" ":" date-time
    mta-name = *text
    mta-name-type = atom
    original-envelope-id-field =
          "Original-Envelope-Id" ":" envelope-id

Moore & Vaudreuil Standards Track [Page 27] RFC 3464 Delivery Status Notifications January 2003

    original-recipient-field =
          "Original-Recipient" ":" address-type ";" generic-address
    per-message-fields =
          [ original-envelope-id-field CRLF ]
          reporting-mta-field CRLF
          [ dsn-gateway-field CRLF ]
          [ received-from-mta-field CRLF ]
          [ arrival-date-field CRLF ]
          *( extension-field CRLF )
    per-recipient-fields =
         [ original-recipient-field CRLF ]
         final-recipient-field CRLF
         action-field CRLF
         status-field CRLF
         [ remote-mta-field CRLF ]
         [ diagnostic-code-field CRLF ]
         [ last-attempt-date-field CRLF ]
         [ final-log-id-field CRLF ]
         [ will-retry-until-field CRLF ]
          *( extension-field CRLF )
    received-from-mta-field =
         "Received-From-MTA" ":" mta-name-type ";" mta-name
    remote-mta-field =
         "Remote-MTA" ":" mta-name-type ";" mta-name
    reporting-mta-field =
          "Reporting-MTA" ":" mta-name-type ";" mta-name
    status-code = DIGIT "." 1*3DIGIT "." 1*3DIGIT
      ; White-space characters and comments are NOT allowed within a
      ; a status-code, though a comment enclosed in parentheses
      ; MAY follow the last numeric sub-field of the status-code.
      ; Each numeric sub-field within the status-code MUST be
      ; expressed without leading zero digits.
    status-field = "Status" ":" status-code
    will-retry-until-field = "Will-Retry-Until" ":" date-time

Moore & Vaudreuil Standards Track [Page 28] RFC 3464 Delivery Status Notifications January 2003

Appendix B - Guidelines for gatewaying DSNs

 NOTE: This section provides non-binding recommendations for the
 construction of mail gateways that wish to provide semi-transparent
 delivery reports between the Internet and another electronic mail
 system.  Specific DSN gateway requirements for a particular pair of
 mail systems may be defined by other documents.

Gatewaying from other mail systems to DSNs

 A mail gateway may issue a DSN to convey the contents of a "foreign"
 delivery or non-delivery notification over Internet mail.  When there
 are appropriate mappings from the foreign notification elements to
 DSN fields, the information may be transmitted in those DSN fields.
 Additional information (such as might be useful in a trouble ticket
 or needed to tunnel the foreign notification through the Internet)
 may be defined in extension DSN fields. (Such fields should be given
 names that identify the foreign mail protocol, e.g., X400-* for X.400
 NDN or DN protocol elements)
 The gateway must attempt to supply reasonable values for the
 Reporting-MTA, Final-Recipient, Action, and Status fields.  These
 will normally be obtained by translating the values from the remote
 delivery or non-delivery notification into their Internet-style
 equivalents.  However, some loss of information is to be expected.
 For example, the set of status-codes defined for DSNs may not be
 adequate to fully convey the delivery diagnostic code from the
 foreign system.  The gateway should assign the most precise code
 which describes the failure condition, falling back on "generic"
 codes such as 2.0.0 (success), 4.0.0 (temporary failure), and 5.0.0
 (permanent failure) when necessary.  The actual foreign diagnostic
 code should be retained in the Diagnostic-Code field (with an
 appropriate diagnostic-type value) for use in trouble tickets or
 tunneling.
 The sender-specified recipient address, and the original envelope-id,
 if present in the foreign transport envelope, should be preserved in
 the Original-Recipient and Original-Envelope-ID fields.
 The gateway should also attempt to preserve the "final" recipient
 addresses and MTA names from the foreign system.  Whenever possible,
 foreign protocol elements should be encoded as meaningful printable
 ASCII strings.
 For DSNs produced from foreign delivery or nondelivery notifications,
 the name of the gateway MUST appear in the DSN-Gateway field of the
 DSN.

Moore & Vaudreuil Standards Track [Page 29] RFC 3464 Delivery Status Notifications January 2003

Gatewaying from DSNs to other mail systems

 It may be possible to gateway DSNs from the Internet into a foreign
 mail system.  The primary purpose of such gatewaying is to convey
 delivery status information in a form that is usable by the
 destination system.  A secondary purpose is to allow "tunneling" of
 DSNs through foreign mail systems, in case the DSN may be gatewayed
 back into the Internet.
 In general, the recipient of the DSN (i.e., the sender of the
 original message) will want to know, for each recipient: the closest
 available approximation to the original recipient address, the
 delivery status (success, failure, or temporary failure), and for
 failed deliveries, a diagnostic code that describes the reason for
 the failure.
 If possible, the gateway should attempt to preserve the Original-
 Recipient address and Original-Envelope-ID (if present), in the
 resulting foreign delivery status report.
 When reporting delivery failures, if the diagnostic-type sub-field of
 the Diagnostic-Code field indicates that the original diagnostic code
 is understood by the destination environment, the information from
 the Diagnostic-Code field should be used.  Failing that, the
 information in the Status field should be mapped into the closest
 available diagnostic code used in the destination environment.
 If it is possible to tunnel a DSN through the destination
 environment, the gateway specification may define a means of
 preserving the DSN information in the delivery status reports used by
 that environment.

Appendix C - Guidelines for use of DSNs by mailing list exploders

 This section pertains only to the use of DSNs by "mailing lists" as
 defined in [4], section 7.2.7.
 DSNs are designed to be used by mailing list exploders to allow them
 to detect and automatically delete recipients for whom mail delivery
 fails repeatedly.
 When forwarding a message to list subscribers, the mailing list
 exploder should always set the envelope return address (e.g., SMTP
 MAIL FROM address) to point to a special address which is set up to
 receive non-delivery reports.  A "smart" mailing list exploder can
 therefore intercept such non-delivery reports, and if they are in the
 DSN format, automatically examine them to determine for which
 recipients a message delivery failed or was delayed.

Moore & Vaudreuil Standards Track [Page 30] RFC 3464 Delivery Status Notifications January 2003

 The Original-Recipient field should be used if available, since it
 should exactly match the subscriber address known to the list.  If
 the Original-Recipient field is not available, the recipient field
 may resemble the list subscriber address.  Often, however, the list
 subscriber will have forwarded his mail to a different address, or
 the address may be subject to some re-writing, so heuristics may be
 required to successfully match an address from the recipient field.
 Care is needed in this case to minimize the possibility of false
 matches.
 The reason for delivery failure can be obtained from the Status and
 Action fields, and from the Diagnostic-Code field (if the status-type
 is recognized).  Reports for recipients with action values other than
 "failed" can generally be ignored; in particular, subscribers should
 not be removed from a list due to "delayed" reports.
 In general, almost any failure status code (even a "permanent" one)
 can result from a temporary condition.  It is therefore recommended
 that a list exploder not delete a subscriber based on any single
 failure DSN (regardless of the status code), but only on the
 persistence of delivery failure over a period of time.
 However, some kinds of failures are less likely than others to have
 been caused by temporary conditions, and some kinds of failures are
 more likely to be noticed and corrected quickly than others.  Once
 more precise status codes are defined, it may be useful to
 differentiate between the status codes when deciding whether to
 delete a subscriber.  For example, on a list with a high message
 volume, it might be desirable to temporarily suspend delivery to a
 recipient address which causes repeated "temporary" failures, rather
 than simply deleting the recipient.  The duration of the suspension
 might depend on the type of error.  On the other hand, a "user
 unknown" error that persisted for several days could be considered a
 reliable indication that address were no longer valid.

Appendix D - IANA registration forms for DSN types

 The forms below are for use when registering a new address-type,
 diagnostic-type, or MTA-name-type with the Internet Assigned Numbers
 Authority (IANA).  Each piece of information requested by a
 registration form may be satisfied either by providing the
 information on the form itself, or by including a reference to a
 published, publicly available specification which includes the
 necessary information.  IANA MAY reject DSN type registrations
 because of incomplete registration forms, imprecise specifications,
 or inappropriate type names.

Moore & Vaudreuil Standards Track [Page 31] RFC 3464 Delivery Status Notifications January 2003

 To register a DSN type, complete the applicable form below and send
 it via Internet electronic mail to <IANA@IANA.ORG>.

IANA registration form for address-type

 A registration for a DSN address-type MUST include the following
 information:
 (a) The proposed address-type name.
 (b) The syntax for mailbox addresses of this type, specified using
     BNF, regular expressions, ASN.1, or other non-ambiguous language.
 (c) If addresses of this type are not composed entirely of graphic
     characters from the US-ASCII repertoire, a specification for how
     they are to be encoded as graphic US-ASCII characters in a DSN
     Original-Recipient or Final-Recipient DSN field.
 (d) [optional] A specification for how addresses of this type are to
     be translated to and from Internet electronic mail addresses.

IANA registration form for diagnostic-type

 A registration for a DSN address-type MUST include the following
 information:
 (a) The proposed diagnostic-type name.
 (b) A description of the syntax to be used for expressing diagnostic
     codes of this type as graphic characters from the US-ASCII
     repertoire.
 (c) A list of valid diagnostic codes of this type and the meaning of
     each code.
 (d) [optional] A specification for mapping from diagnostic codes of
     this type to DSN status codes (as defined in [5]).

IANA registration form for MTA-name-type

 A registration for a DSN MTA-name-type must include the following
 information:
 (a) The proposed MTA-name-type name.
 (b) A description of the syntax of MTA names of this type, using BNF,
     regular expressions, ASN.1, or other non-ambiguous language.

Moore & Vaudreuil Standards Track [Page 32] RFC 3464 Delivery Status Notifications January 2003

 (c) If MTA names of this type do not consist entirely of graphic
     characters from the US-ASCII repertoire, a specification for how
     an MTA name of this type should be expressed as a sequence of
     graphic US-ASCII characters.

Appendix E - Examples

 These examples are provided as illustration only, and are not
 considered part of the DSN protocol specification.  If an example
 conflicts with the protocol definition above, the example is wrong.
 Likewise, the use of *-type sub-field names or extension fields in
 these examples is not to be construed as a definition for those type
 names or extension fields.
 These examples were manually translated from bounced messages using
 whatever information was available.

Moore & Vaudreuil Standards Track [Page 33] RFC 3464 Delivery Status Notifications January 2003

Simple DSN

 This is a simple DSN issued after repeated attempts to deliver a
 message failed.  In this case, the DSN is issued by the same MTA from
 which the message was originated.
 Date: Thu, 7 Jul 1994 17:16:05 -0400 From: Mail Delivery Subsystem
 <MAILER-DAEMON@CS.UTK.EDU> Message-Id:
 <199407072116.RAA14128@CS.UTK.EDU> Subject: Returned mail: Cannot
 send message for 5 days To: <owner-info-mime@cs.utk.edu> MIME-
 Version: 1.0 Content-Type: multipart/report; report-type=delivery-
 status;
        boundary="RAA14128.773615765/CS.UTK.EDU"
  1. -RAA14128.773615765/CS.UTK.EDU
 The original message was received at Sat, 2 Jul 1994 17:10:28 -0400
 from root@localhost
  1. —- The following addresses had delivery problems —–

louisl@larry.slip.umd.edu (unrecoverable error)

  1. —- Transcript of session follows —–

louisl@larry.slip.umd.edu… Deferred: Connection timed out

             with larry.slip.umd.edu.
 Message could not be delivered for 5 days
 Message will be deleted from queue
  1. -RAA14128.773615765/CS.UTK.EDU

content-type: message/delivery-status

 Reporting-MTA: dns; cs.utk.edu
 Original-Recipient: rfc822;louisl@larry.slip.umd.edu
 Final-Recipient: rfc822;louisl@larry.slip.umd.edu
 Action: failed
 Status: 4.0.0
 Diagnostic-Code: smtp; 426 connection timed out
 Last-Attempt-Date: Thu, 7 Jul 1994 17:15:49 -0400
  1. -RAA14128.773615765/CS.UTK.EDU

content-type: message/rfc822

 [original message goes here]
  1. -RAA14128.773615765/CS.UTK.EDU–

Moore & Vaudreuil Standards Track [Page 34] RFC 3464 Delivery Status Notifications January 2003

Multi-Recipient DSN

 This is another DSN issued by the sender's MTA, which contains
 details of multiple delivery attempts.  Some of these were detected
 locally, and others by a remote MTA.
 Date: Fri, 8 Jul 1994 09:21:47 -0400
 From: Mail Delivery Subsystem <MAILER-DAEMON@CS.UTK.EDU>
 Subject: Returned mail: User unknown
 To: <owner-ups-mib@CS.UTK.EDU>
 MIME-Version: 1.0
 Content-Type: multipart/report; report-type=delivery-status;
        boundary="JAA13167.773673707/CS.UTK.EDU"
  1. -JAA13167.773673707/CS.UTK.EDU

content-type: text/plain; charset=us-ascii

  1. —- The following addresses had delivery problems —–

arathib@vnet.ibm.com (unrecoverable error)

 <wsnell@sdcc13.ucsd.edu> (unrecoverable error)
  1. -JAA13167.773673707/CS.UTK.EDU

content-type: message/delivery-status

 Reporting-MTA: dns; cs.utk.edu
 Original-Recipient: rfc822;arathib@vnet.ibm.com
 Final-Recipient: rfc822;arathib@vnet.ibm.com
 Action: failed
 Status: 5.0.0 (permanent failure)
 Diagnostic-Code: smtp;  550 'arathib@vnet.IBM.COM' is not a
  registered gateway user
 Remote-MTA: dns; vnet.ibm.com
 Original-Recipient: rfc822;johnh@hpnjld.njd.hp.com
 Final-Recipient: rfc822;johnh@hpnjld.njd.hp.com
 Action: delayed
 Status: 4.0.0 (hpnjld.njd.jp.com: host name lookup failure)
 Original-Recipient: rfc822;wsnell@sdcc13.ucsd.edu
 Final-Recipient: rfc822;wsnell@sdcc13.ucsd.edu
 Action: failed
 Status: 5.0.0
 Diagnostic-Code: smtp; 550 user unknown
 Remote-MTA: dns; sdcc13.ucsd.edu
  1. -JAA13167.773673707/CS.UTK.EDU

content-type: message/rfc822

Moore & Vaudreuil Standards Track [Page 35] RFC 3464 Delivery Status Notifications January 2003

  [original message goes here]
  1. -JAA13167.773673707/CS.UTK.EDU–

DSN from gateway to foreign system

 A delivery report generated by Message Router (MAILBUS) and gatewayed
 by PMDF_MR to a DSN.  In this case the gateway did not have
 sufficient information to supply an original-recipient address.
 Disclose-recipients: prohibited
 Date: Fri, 08 Jul 1994 09:21:25 -0400 (EDT)
 From: Message Router Submission Agent <AMMGR@corp.timeplex.com>
 Subject: Status of: Re: Battery current sense
 To: owner-ups-mib@CS.UTK.EDU
 Message-id: <01HEGJ0WNBY28Y95LN@mr.timeplex.com>
 MIME-version: 1.0
 content-type: multipart/report;
     report-type=delivery-status;
     boundary="84229080704991.122306.SYS30"
  1. -84229080704991.122306.SYS30

content-type: text/plain

 Invalid address - nair_s
 %DIR-E-NODIRMTCH, No matching Directory Entry
 Entry found
  1. -84229080704991.122306.SYS30

content-type: message/delivery-status

 Reporting-MTA: mailbus; SYS30
 Final-Recipient: unknown; nair_s
 Status: 5.0.0 (unknown permanent failure)
 Action: failed
  1. -84229080704991.122306.SYS30–

Moore & Vaudreuil Standards Track [Page 36] RFC 3464 Delivery Status Notifications January 2003

Delayed DSN

 A delay report from a multiprotocol MTA.  Note that there is no
 returned content, so no third body part appears in the DSN.
 MIME-Version: 1.0
 From: <postmaster@nsfnet-relay.ac.uk>
 Message-Id: <199407092338.TAA23293@CS.UTK.EDU>
 Received: from nsfnet-relay.ac.uk by sun2.nsfnet-relay.ac.uk
         id <g.12954-0@sun2.nsfnet-relay.ac.uk>;
         Sun, 10 Jul 1994 00:36:51 +0100
 To: owner-info-mime@cs.utk.edu
 Date: Sun, 10 Jul 1994 00:36:51 +0100
 Subject: WARNING: message delayed at "nsfnet-relay.ac.uk"
 content-type: multipart/report; report-type=delivery-status;
        boundary=foobar
  1. -foobar

content-type: text/plain

 The following message:
 UA-ID: Reliable PC (...
 Q-ID: sun2.nsf:77/msg.11820-0
 has not been delivered to the intended recipient:
     thomas@de-montfort.ac.uk
 despite repeated delivery attempts over the past 24 hours.
 The usual cause of this problem is that the remote system is
 temporarily unavailable.
 Delivery will continue to be attempted up to a total elapsed time of
 168 hours, i.e., 7 days.
 You will be informed if delivery proves to be impossible within this
 time.
 Please quote the Q-ID in any queries regarding this mail.
  1. -foobar

content-type: message/delivery-status

 Reporting-MTA: dns; sun2.nsfnet-relay.ac.uk
 Final-Recipient: rfc822;thomas@de-montfort.ac.uk

Moore & Vaudreuil Standards Track [Page 37] RFC 3464 Delivery Status Notifications January 2003

 Status: 4.0.0 (unknown temporary failure)
 Action: delayed
  1. -foobar–

Appendix F - Changes from RFC 1894

 Changed Authors contact information
 Updated required standards boilerplate
 Edited the text to make it spell-checker and grammar checker
 compliant
 Updated references to point to later, more mature documents, changed
 reference enumeration scheme.
 Fixed paragraph numbering on page 20
 Fixed Delayed DSN example
 Added Table of Contents
 Moved Appendices to the end of the document
 Changed the MTA-name-Type for gateways into Internet mail, the
 MTA-name-type from "SMTP" to "dns".

Moore & Vaudreuil Standards Track [Page 38] RFC 3464 Delivery Status Notifications January 2003

Authors' Addresses

 Keith Moore
 University of Tennessee
 1122 Volunteer Blvd, Suite 203
 Knoxville TN 37996-3450
 USA
 Phone: +1-865-974-3126
 Fax:   +1-865-974-8296
 EMail: moore@cs.utk.edu
 Gregory M. Vaudreuil
 Lucent Technologies
 7291 Williamson Rd
 Dallas, Tx. 75214
 USA
 Phone: +1 214 823 9325
 EMail: GregV@ieee.org

Moore & Vaudreuil Standards Track [Page 39] RFC 3464 Delivery Status Notifications January 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Moore & Vaudreuil Standards Track [Page 40]

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