Network Working Group                                           K. Moore
Request for Comments: 1894                       University of Tennessee
Category: Standards Track                                   G. Vaudreuil
                                                  Octel Network Services
                                                            January 1996

     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.


   This memo defines a 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 "LAN-based"
   systems), the 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.

   Any questions, comments, and reports of defects or ambiguities in
   this specification may be sent to the mailing list for the NOTARY
   working group of the IETF, using the address
   .  Requests to subscribe to the mailing
   list should be addressed to .
   Implementors of this specification are encouraged to subscribe to the
   mailing list, so that they will quickly be informed of any problems
   which might hinder interoperability.

   NOTE: This document is a Proposed Standard.  If and when this
   protocol is submitted for Draft Standard status, any normative text
   (phrases containing SHOULD, SHOULD NOT, MUST, MUST NOT, or MAY) in
   this document will be re-evaluated in light of implementation

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   experience, and are thus subject to change.

1. Introduction

   This memo defines a MIME [1] 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 [2].

   This memo defines only the format of the notifications.  An extension
   to the Simple Message Transfer Protocol (SMTP) [3] to fully support
   such notifications is the subject of a separate memo [4].

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 which is largely independent of human language;

(b) Allow mail user agents to keep track of the delivery status of
    messages sent, by associating returned DSNs with earlier message

(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

(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

(f) Allow language-independent, yet reasonably precise, indications of
    the reason for the failure of a message to be delivered (once status
    codes of sufficient precision are defined); 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

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    sender wants to report the problem to the remote MTA administrator.

1.2 Requirements

    These purposes place the following constraints on the notification

(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

(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.

   A DSN contains a set of per-message fields which 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.

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   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

   (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 delivered.  In this case the relaying
   MTA is the Reporting MTA, and the "next hop" MTA is known as the
   Remote MTA.

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Figure 1 illustrates the relationship between the various MTAs.

+-----+    +--------+           +---------+    +---------+      +------+
|     |    |        |           |Received-|    |         |      |      |
|     | => |Original| => ... => |  From   | => |Reporting| ===> |Remote|
| user|    |   MTA  |           |   MTA   |    |   MTA   | ".

   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 be still be issued in response to a single message

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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
     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 [6]).  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 which 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 [7].

   A number of DSN fields are defined to have a portion of a field body
   of "xtext".  "xtext" is used to allow encoding sequences of octets
   which contain values outside the range [1-127 decimal] of traditional
   ASCII characters, and also to allow comments to be inserted in the
   data.  Any octet may be encoded as "+" followed by two upper case

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   hexadecimal digits.  (The "+" character MUST be encoded as "+2B".)
   With certain exceptions, octets that correspond to ASCII characters
   may be represented as themselves.  SPACE and HTAB characters are
   ignored.  Comments may be included by enclosing them in parenthesis.
   Except within comments, encoded-words such as defined in [7] may NOT
   be used in xtext.

   "xtext" is formally defined as follows:

     xtext = *( xchar / hexchar / linear-white-space / comment )

     xchar = any ASCII CHAR between "!" (33) and "~" (126) inclusive,
          except for "+", "\" and "(".

   "hexchar"s are intended to encode octets that cannot be represented
   as plain text, either because they are reserved, or because they are
   non-printable.  However, any octet value may be represented by a

     hexchar = ASCII "+" immediately followed by two upper case
          hexadecimal digits

   When encoding an octet sequence as xtext:

   + Any ASCII CHAR between "!" and "~" inclusive, except for "+", "\",
     and "(", MAY be encoded as itself.  (Some CHARs in this range may
     also be encoded as "hexchar"s, at the implementor's discretion.)

   + ASCII CHARs that fall outside the range above must be encoded as

   + Line breaks (CR LF SPACE) MAY be inserted as necessary to keep line
     lengths from becoming excessive.

   + Comments MAY be added to clarify the meaning for human readers.

2.1.2 "*-type" subfields

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

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   The "-type" subfields 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 [3], the "smtp" diagnostic-type is

         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 a one or more specifications that provide
   such descriptions.  (The "rfc822" address-type, "smtp" diagnostic-
   type, and "dns" MTA-name-type are defined in [4].)  Registration
   forms for address-type, diagnostic-type, and MTA-name-type appear in
   section 8 of this document.

   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 [6], 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

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2.2 Per-Message DSN Fields

   Some fields of a DSN apply to all of the delivery attempts described
   by that DSN.  These fields may appear at most 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" which 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 which
   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
   which 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.

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   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 which 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 nondelivery 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.

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sender's environment                            recipient's environment
............................ ..........................................
                           : :
                       (1) : :                             (2)
  +-----+  +--------+  +--------+  +---------+  +---------+   +------+
  |     |  |        |  |        |  |Received-|  |         |   |      |
  |     |=>|Original|=>|        |->|  From   |->|Reporting|-->|Remote|
  | user|  |   MTA  |  |        |  |   MTA   |  |   MTA   |") (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

   Implementors are cautioned that many existing MTAs will send
   nondelivery 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.

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.

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   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.[9] 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

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5. Appendix - 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 [8].

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

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

original-recipient-field =
     "Original-Recipient" ":" address-type ";" generic-address

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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 ]
     [ 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
     ; status-code, though a comment enclosed in parentheses MAY follow
     ; the last numeric subfield of the status-code.  Each numeric
     ; subfield 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

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6. Appendix - 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.

6.1 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

   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

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6.2 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 subfield 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.

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7. Appendix - Guidelines for use of DSNs by mailing list exploders

   NOTE: 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
   received nondelivery reports.  A "smart" mailing list exploder can
   therefore intercept such nondelivery reports, and if they are in the
   DSN format, automatically examine them to determine for which
   recipients a message delivery failed or was delayed.

   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

   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

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   might depend on the type of error.  On the other hand, a "user
   unknown" error which persisted for several days could be considered a
   reliable indication that address were no longer valid.

8. Appendix - 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.

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

8.1 IANA registration form for address-type

   A registration for a DSN address-type MUST include the following

(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.

8.2 IANA registration form for diagnostic-type

   A registration for a DSN address-type MUST include the following

(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

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(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]).

8.3 IANA registration form for MTA-name-type

   A registration for a DSN MTA-name-type must include the following

(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.

(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.

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9. Appendix - Examples

   NOTE:  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 subfield 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.

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9.1  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 
   Message-Id: <199407072116.RAA14128@CS.UTK.EDU>
   Subject: Returned mail: Cannot send message for 5 days
   MIME-Version: 1.0
   Content-Type: multipart/report; report-type=delivery-status;


   The original message was received at Sat, 2 Jul 1994 17:10:28 -0400
   from root@localhost

      ----- The following addresses had delivery problems -----
     (unrecoverable error)

      ----- Transcript of session follows -----
   ... Deferred: Connection timed out
   Message could not be delivered for 5 days
   Message will be deleted from queue

   content-type: message/delivery-status

   Reporting-MTA: dns;

   Original-Recipient: rfc822;
   Final-Recipient: rfc822;
   Action: failed
   Status: 4.0.0
   Diagnostic-Code: smtp; 426 connection timed out
   Last-Attempt-Date: Thu, 7 Jul 1994 17:15:49 -0400

   content-type: message/rfc822

   [original message goes here]

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9.2  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 
   Subject: Returned mail: User unknown
   MIME-Version: 1.0
   Content-Type: multipart/report; report-type=delivery-status;

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

      ----- The following addresses had delivery problems -----
     (unrecoverable error)
     (unrecoverable error)

   content-type: message/delivery-status

   Reporting-MTA: dns;

   Original-Recipient: rfc822;
   Final-Recipient: rfc822;
   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;

   Original-Recipient: rfc822;
   Final-Recipient: rfc822;
   Action: delayed
   Status: 4.0.0 ( host name lookup failure)

   Original-Recipient: rfc822;
   Final-Recipient: rfc822;
   Action: failed
   Status: 5.0.0
   Diagnostic-Code: smtp; 550 user unknown
   Remote-MTA: dns;

   content-type: message/rfc822

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   [original message goes here]

9.3 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 
   Subject: Status of : Re: Battery current sense
   To: owner-ups-mib@CS.UTK.EDU
   Message-id: <>
   MIME-version: 1.0
   content-type: multipart/report; report-type=delivery-status;

   content-type: text/plain

   Invalid address - nair_s
   %DIR-E-NODIRMTCH, No matching Directory Entry found

   content-type: message/delivery-status

   Reporting-MTA: mailbus; SYS30

   Final-Recipient: unknown; nair_s
   Status: 5.0.0 (unknown permanent failure)
   Action: failed


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9.4 A delay report from a multiprotocol MTA.  Note that there is no
    returned content, so no third body part appears in the DSN.

   Message-Id: <199407092338.TAA23293@CS.UTK.EDU>
   Received: from by
             id ;
   Sun, 10 Jul 1994 00:36:51 +0100
   Date: Sun, 10 Jul 1994 00:36:51 +0100
   Subject: WARNING: message delayed at ""
   content-type: multipart/report; report-type=delivery-status;

   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:

   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, ie 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.

   content-type: message/delivery-status

   Reporting-MTA: dns;

   Final-Recipient: rfc822;
   Status: 4.0.0 (unknown temporary failure)
   Action: delayed

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10. Acknowledgments

   The authors wish to thank the following people for their reviews of
   earlier drafts of this document 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

11. References

[1] Borenstein, N., Freed, N. "Multipurpose Internet Mail Extensions",
    RFC 1521, Bellcore, Innosoft, September 1993.

[2] Vaudreuil, G., "The Multipart/Report Content Type for the Reporting
    of Mail System Administrative Messages", RFC 1892, Octal Network
    Services, January 1996.

[3] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
    USC/Information Sciences Institute, August 1982.

[4] Moore, K., "SMTP Service Extension for Delivery Status
    Notifications", RFC 1891, University of Tennessee, January 1996.

[5] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893, Octal
    Network Services, January 1996.

[6] Crocker, D., "Standard for the Format of ARPA Internet Text
    Messages", STD 11, RFC 822, UDEL, August 1982.

[7] Moore, K. "MIME (Multipurpose Internet Mail Extensions) Part Two:
    Message Header Extensions for Non-Ascii Text", RFC 1522, University
    of Tennessee, September 1993.

[8] Braden, R. (ed.)  "Requirements for Internet Hosts - Application and
    Support", STD 3, RFC 1123, USC/Information Sciences Institute,
    October 1989.

[9] Partridge, C., "Duplicate Messages and SMTP", RFC 1047, BBN,
    February 1988.

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11. Authors' Addresses

   Keith Moore
   University of Tennessee
   107 Ayres Hall
   Knoxville, TN 37996-1301

   Phone: +1 615 974 3126
   Fax: +1 615 974 8296

   Gregory M. Vaudreuil
   Octel Network Services
   17080 Dallas Parkway
   Dallas, TX 75248-1905

   EMail: Greg.Vaudreuil@Octel.Com

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