    Network Working Group                                          E. Allman
    Request for Comments: 3886                                Sendmail, Inc.
    Updates: 3463                                             September 2004
    Category: Standards Track

          An Extensible Message Format for Message Tracking Responses

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2004).

    Abstract

       Message Tracking is expected to be used to determine the status of
       undelivered e-mail upon request.  Tracking is used in conjunction
       with Delivery Status Notifications (DSN) and Message Disposition
       Notifications (MDN); generally, a message tracking request will be
       issued only when a DSN or MDN has not been received within a
       reasonable timeout period.

       This memo defines a MIME content-type for message tracking status in
       the same spirit as RFC 3464, "An Extensible Message Format for
       Delivery Status Notifications".  It is to be issued upon a request as
       described in "Message Tracking Query Protocol".  This memo defines
       only the format of the status information.  An extension to SMTP to
       label messages for further tracking and request tracking status is
       defined in a separate memo.

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    RFC 3886                Message/Tracking-Status           September 2004

    1.  Introduction

       Message Tracking is expected to be used to determine the status of
       undelivered e-mail upon request.  Tracking is used in conjunction
       with Delivery Status Notifications (DSN) [RFC-DSN-SMTP] and Message
       Disposition Notifications (MDN) [RFC-MDN]; generally, a message
       tracking request will be issued only when a DSN or MDN has not been
       received within a reasonable timeout period.

       This memo defines a MIME [RFC-MIME] content-type for message tracking
       status in the same spirit as RFC 3464, "An Extensible Message Format
       for Delivery Status Notifications" [RFC-DSN-STAT].  It is to be
       issued upon a request as described in "Message Tracking Query
       Protocol" [RFC-MTRK-MTQP].  This memo defines only the format of the
       status information.  An extension to SMTP [RFC-ESMTP] to label
       messages for further tracking and request tracking status is defined
       in a separate memo [RFC-MTRK-SMTPEXT].

    2.  Other Documents and Conformance

       The model used for Message Tracking is described in [RFC-MTRK-MODEL].

       Message tracking is intended for use as a "last resort" mechanism.
       Normally, Delivery Status Notifications (DSNs) [RFC-DSN-SMTP] and
       Message Disposition Notifications (MDNs) [RFC-MDN] would provide the
       primary delivery status.  Only if no response is received from either
       of these mechanisms would Message Tracking be used.

       This document is based on [RFC-DSN-STAT].  Sections 1.3
       (Terminology), 2.1.1 (General conventions for DSN fields), 2.1.2
       ("*-type" subfields), and 2.1.3 (Lexical tokens imported from RFC
       822) of [RFC-DSN-STAT] are included into this document by reference.
       Other sections are further incorporated as described herein.

       Syntax notation in this document conforms to [RFC-ABNF].

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

       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 RFC 2119 [RFC-
       KEYWORDS].

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    RFC 3886                Message/Tracking-Status           September 2004

    3.  Format of a Message Tracking Status Notification

       A Message Tracking Status Notification (MTSN) is intended to be
       returned as the body of a Message Tracking request [RFC-MTRK-MTQP].
       The actual body MUST be a multipart/related [RFC-RELATED] with type
       parameter of "message/tracking-status"; each subpart MUST be of type
       "message/tracking-status" as described herein.  The multipart/related
       body can include multiple message/tracking-status parts if an MTQP
       server chains requests to the next server; see [RFC-MTRK-MODEL] and
       [RFC-MTRK-MTQP] for more information about chaining.

    3.1.  The message/tracking-status content-type

       The message/tracking-status content-type is defined as follows:

       MIME type name:           message
       MIME subtype name:        tracking-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 body of a message/tracking-status is modeled after [RFC-DSN-
       STAT].  That body consists of one or more "fields" formatted to
       according to the ABNF of RFC 2822 header "fields" (see [RFC-MSGFMT]).
       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.  Note that there will be a blank line between the final
       per-recipient field and the MIME boundary, since one CRLF is
       necessary to terminate the field, and a second is necessary to
       introduce the MIME boundary.  Formally, the syntax of the
       message/tracking-status content is as follows:

       tracking-status-content =
                 per-message-fields 1*( CRLF per-recipient-fields )

       The per-message fields are described in section 3.2.  The per-
       recipient fields are described in section 3.3.

    3.1.1.  General conventions for MTSN fields

       Section 2.1.1 (General conventions for DSN fields) of [RFC-DSN-STAT]
       is included herein by reference.  Notably, the definition of xtext is
       identical to that of that document.

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    RFC 3886                Message/Tracking-Status           September 2004

    3.1.2.  *-type subfields

       Section 2.1.2 (*-type subfields) of [RFC-DSN-STAT] is included herein
       by reference.  Notably, the definitions of address-type, diagnostic-
       type, and MTA-name type are identical to that of RFC 3464.

    3.2.  Per-Message MTSN Fields

       Some fields of an MTSN apply to all of the addresses in a single
       envelope.  These fields may appear at most once in any MTSN.  These
       fields are used to correlate the MTSN 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
                    arrival-date-field CRLF
                    *( extension-field CRLF )

    3.2.1.  The Original-Envelope-Id field

       The Original-Envelope-Id field is defined as in section 2.2.1 of
       [RFC-DSN-STAT].  This field is REQUIRED.

    3.2.2.  The Reporting-MTA field

       The Reporting-MTA field is defined as in section 2.2.2 of [RFC-DSN-
       STAT].  This field is REQUIRED.

    3.2.3.  The Arrival-Date field

       The Arrival-Date field is defined as in section 2.2.5 of [RFC-DSN-
       STAT].  This field is REQUIRED.

    3.3.  Per-Recipient MTSN fields

       An MTSN 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.

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       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 ]
                    [ last-attempt-date-field CRLF ]
                    [ will-retry-until-field CRLF ]
                    *( extension-field CRLF )

    3.3.1.  Original-Recipient field

       The Original-Recipient field is defined as in section 2.3.1 of [RFC-
       DSN-STAT].  This field is REQUIRED.

    3.3.2.  Final-Recipient field

       The required Final-Recipient field is defined as in section 2.3.2 of
       [RFC-DSN-STAT].  This field is REQUIRED.

    3.3.3.  Action field

       The required 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 MTSN.  The syntax is as defined in RFC 3464.
       This field is REQUIRED.

       Valid actions are:

       failed       The message could not be delivered.  If DSNs have been
                    enabled, a "failed" DSN should already have been
                    returned.

       delayed      The message is currently waiting in the MTA queue for
                    future delivery.  Essentially, this action means "the
                    message is located, and it is here."

       delivered    The message has been successfully delivered to the final
                    recipient.  This includes "delivery" to a mailing list
                    exploder.  It does not indicate that the message has
                    been read.  No further information is available; in
                    particular, the tracking agent SHOULD NOT attempt
                    further "downstream" tracking requests.

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    RFC 3886                Message/Tracking-Status           September 2004

       expanded     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.  However,
                    these additional addresses are not trackable, and the
                    tracking agent SHOULD NOT attempt further "downstream"
                    tracking requests.

       relayed      The message has been delivered into an environment that
                    does not support message tracking.  No further
                    information is available; in particular, the tracking
                    agent SHOULD NOT attempt further "downstream" tracking
                    requests.

       transferred  The message has been transferred to another MTRK-
                    compliant MTA.  The tracking agent SHOULD attempt
                    further "downstream" tracking requests unless that
                    information is already given in a chaining response.

       opaque       The message may or may not have been seen by this
                    system.  No further information is available or
                    forthcoming.

       There may be some confusion between when to use "expanded" versus
       "delivered".  Whenever possible, "expanded" should be used when the
       MTA knows that the message will be sent to multiple addresses.
       However, in some cases the delivery occurs to a program which,
       unknown to the MTA, causes mailing list expansion; in the extreme
       case, the delivery may be to a real mailbox that has the side effect
       of list expansion.  If the MTA cannot ensure that this delivery will
       cause list expansion, it should set the action to "delivered".

    3.3.4.  Status field

       The Status field is defined as in RFC 3464.  A new code is added to
       RFC 3463 [RFC-EMSSC], "Enhanced Mail System Status Codes",

          X.1.9   Message relayed to non-compliant mailer"

             The mailbox address specified was valid, but the message has
             been relayed to a system that does not speak this protocol; no
             further information can be provided.

       A 2.1.9 Status field MUST be used exclusively with a "relayed" Action
       field.  This field is REQUIRED.

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    RFC 3886                Message/Tracking-Status           September 2004

    3.3.5.  Remote-MTA field

       The Remote-MTA field is defined as in section Reference 2.3.5 of
       [RFC-DSN-STAT].  This field MUST NOT be included if no delivery
       attempts have been made or if the Action field has value "opaque".
       If delivery to some agent other than an MTA (for example, a Local
       Delivery Agent) then this field MAY be included, giving the name of
       the host on which that agent was contacted.

    3.3.6.  Last-Attempt-Date field

       The Last-Attempt-Date field is defined as in section Reference 2.3.7
       of [RFC-DSN-STAT].  This field is REQUIRED if any delivery attempt
       has been made and the Action field does not have value "opaque", in
       which case it will specify when it last attempted to deliver this
       message to another MTA or other Delivery Agent.  This field MUST NOT
       be included if no delivery attempts have been made.

    3.3.7.  Will-Retry-Until field

       The Will-Retry-Until field is defined as in section Reference 2.3.9
       of [RFC-DSN-STAT].  If the message is not in the local queue or the
       Action field has the value "opaque" the Will-Retry-Until field MUST
       NOT be included; otherwise, this field SHOULD be included.

    3.4.  Extension fields

       Future extension fields may be defined as defined in section 2.4 of
       [RFC-DSN-STAT].

    3.5.  Interaction Between MTAs and LDAs

       A message that has been delivered to a Local Delivery Agent (LDA)
       that understands message tracking (in particular, an LDA speaking
       LMTP [RFC-LMTP] that supports the MTRK extension) SHOULD pass the
       tracking request to the LDA.  In this case, the Action field for the
       MTA->LDA exchange will look the same as a transfer to a compliant
       MTA; that is, a "transferred" tracking status will be issued.

    4.  Security Considerations

    4.1.  Forgery

       Malicious servers may attempt to subvert message tracking and return
       false information.  This could result in misdirection or
       misinterpretation of results.

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    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:

       (a)  respond with a "relayed" tracking status when a message is
            forwarded to a confidential forwarding address, and disabling
            further message tracking requests.

       (b)  declaring the message to be delivered, issuing a "delivered"
            tracking status, re-sending the message to the confidential
            forwarding address, and disabling further message tracking
            requests.

       The tracking algorithms MUST NOT allow tracking through list
       expansions.  When a message is delivered to a list, a tracking
       request MUST respond with an "expanded" tracking status and MUST NOT
       display the contents of the list.

    5.  IANA Considerations

       IANA has registered the SMTP extension defined in section 3.

    6.  Acknowledgements

       Several individuals have commented on and enhanced this document,
       including Tony Hansen, Philip Hazel, Alexey Melnikov, Lyndon
       Nerenberg, Chris Newman, Gregory Neil Shapiro, and Dan Wing.

    7.  References

    7.1.  Normative References

       [RFC-MTRK-MODEL]     Hansen, T., "Message Tracking Model and
                            Requirements", RFC 3888, September 2004.

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    RFC 3886                Message/Tracking-Status           September 2004

       [RFC-MTRK-MTQP]      Hansen, T., "Message Tracking Query Protocol",
                            RFC 3887, September 2004.

       [RFC-MTRK-SMTPEXT]   Allman, E., "SMTP Service Extension for Message
                            Tracking", RFC 3885, September 2004.

       [RFC-ABNF]           Crocker, D., Ed. and P. Overell, "Augmented BNF
                            for Syntax Specifications: ABNF", RFC 2234,
                            November 1997.

       [RFC-EMSSC]          Vaudreuil, G., "Enhanced Mail System Status
                            Codes", RFC 3463, January 2003.

       [RFC-HOSTREQ]        Braden, R., Ed., "Requirements for Internet
                            Hosts -- Application and Support", STD 3, RFC
                            1123, October 1989.

       [RFC-KEYWORDS]       Bradner, S., "Key words for use in RFCs to
                            Indicate Requirement Levels", BCP 14, RFC 2119,
                            March 1997.

       [RFC-MIME]           Freed, N. and N. Borenstein, "Multipurpose
                            Internet Mail Extensions (MIME) Part One: Format
                            of Internet Message Bodies", RFC 2045, November
                            1996.

       [RFC-MSGFMT]         Resnick, P., Ed., "Internet Message Format", RFC
                            2822, April 2001.

       [RFC-RELATED]        Levinson, E., "The MIME Multipart/Related
                            Content-type", RFC 2387, August 1998.

    7.2.  Informational References

       [RFC-DSN-SMTP]       Moore, K., "Simple Mail Transfer Protocol (SMTP)
                            Service Extension for Delivery Status
                            Notifications (DSNs)", RFC 3461, January 2003.

       [RFC-DSN-STAT]       Moore, K. and G. Vaudreuil, "An Extensible
                            Message Format for Delivery Status
                            Notifications", RFC 3464, January 2003.

       [RFC-ESMTP]          Rose, M., Stefferud, E., Crocker, D., Klensin,
                            J., and N. Freed, "SMTP Service Extensions", STD
                            10, RFC 1869, November 1995.

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    RFC 3886                Message/Tracking-Status           September 2004

       [RFC-LMTP]           Myers, J., "Local Mail Transfer Protocol", RFC
                            2033, October 1996.

       [RFC-MDN]            Hansen, T. and G. Vaudreuil, Eds., "Message
                            Disposition Notifications", RFC 3798, May 2004.

    8.  Author's Address

       Eric Allman
       Sendmail, Inc.
       6425 Christie Ave, 4th Floor
       Emeryville, CA  94608
       U.S.A.

       Phone: +1 510 594 5501
       Fax:   +1 510 594 5429
       EMail: eric@Sendmail.COM

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    9. Full Copyright Statement

       Copyright (C) The Internet Society (2004).

       This document is subject to the rights, licenses and restrictions
       contained in BCP 78, and except as set forth therein, the authors
       retain all their rights.

       This document and the information contained herein are provided on an
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       REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
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       WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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    Acknowledgement

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

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