SMTP Service Extension for Message Tracking
RFC 3885

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Network Working Group                                          E. Allman
Request for Comments: 3885                                Sendmail, Inc.
Updates: 3461                                                  T. Hansen
Category: Standards Track                              AT&T Laboratories
                                                          September 2004

                         SMTP Service Extension
                          for Message Tracking

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This memo defines an extension to the SMTP service whereby a client
   may mark a message for future tracking.

1.  Other Documents and Conformance

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

   Doing a Message Tracking query is intended 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 the message is not
   received, or there is no response from either of these mechanisms
   should a Message Tracking query be issued.

   The definition of the base64 token is imported from section 6.8 of
   [RFC-MIME].  Formally,

      base64 =  %x2b / %x2f / %x30-39 / %x41-5a / %x61-7a

Allman & Hansen             Standards Track                     [Page 1]
RFC 3885            Message Tracking ESMTP Extension      September 2004

   The definition of the DIGIT token is imported from [RFC-MSGFMT].
   Formally,

      DIGIT =        %x30-39

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

   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
   [RFC-KEYWORDS].

2.  SMTP Extension Overview

   The Message Tracking SMTP service extension uses the SMTP service
   extension mechanism described in [RFC-ESMTP].  The following service
   extension is hereby defined:

   (1)   The name of the SMTP service extension is "Message Tracking".

   (2)   The EHLO keyword value associated with this extension is
         "MTRK".

   (3)   No parameters are allowed with this EHLO keyword value.  Future
         documents may extend this specification by specifying
         parameters to this keyword value.

   (4)   One optional parameter using the keyword "MTRK" is added to the
         MAIL command.  In addition, the ENVID parameter of the MAIL
         command (as defined in RFC 3461) MUST be supported, with
         extensions as described below.  The ORCPT parameter of the RCPT
         command (as defined in RFC 3461) MUST also be supported.  All
         semantics associated with ENVID and ORCPT described in RFC 3461
         MUST be supported as part of this extension.

   (5)   The maximum length of a MAIL command line is increased by 40
         characters by the possible addition of the MTRK keyword and
         value.  Note that the 507 character extension of RCPT commands
         for the ORCPT parameter and the 107 character extension of MAIL
         commands for the ENVID parameter as mandated by RFC 3461 [RFC-
         DSN-SMTP] must also be included.

   (6)   No SMTP verbs are defined by this extension.

Allman & Hansen             Standards Track                     [Page 2]
RFC 3885            Message Tracking ESMTP Extension      September 2004

3.  The Extended MAIL Command

   The extended MAIL command is issued by an SMTP client when it wishes
   to inform an SMTP server that message tracking information should be
   retained for future querying.  The extended MAIL command is identical
   to the MAIL command as defined in [RFC-SMTP], except that MTRK,
   ORCPT, and ENVID parameters appear after the address.

3.1.  The MTRK parameter to the ESMTP MAIL command

   Any sender wishing to request the retention of data for further
   tracking of message must first tag that message as trackable by
   creating two values A and B:

      A = some-large-random-number
      B = SHA1(A)

   The large random number A is calculated on a host-dependent basis.
   See [RFC-RANDOM] for a discussion of choosing good random numbers.
   This random number MUST be at least 128 bits but MUST NOT be more
   than 1024 bits.

   The 128-bit hash B of A is then computed using the SHA-1 algorithm as
   described in [NIST-SHA1].

   The sender then base64 encodes value B and passes that value as the
   mtrk-certifier on the MAIL command:

      mtrk-parameter  = "MTRK=" mtrk-certifier [ ":" mtrk-timeout ]