Indicating Email Handling States in Trace Fields
RFC 6729

Document Type RFC - Proposed Standard (September 2012; No errata)
Authors Dave Crocker  , Murray Kucherawy 
Last updated 2020-07-29
Replaces draft-kucherawy-received-state
Stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Alexey Melnikov
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Internet Engineering Task Force (IETF)                        D. Crocker
Request for Comments: 6729                   Brandenburg InternetWorking
Category: Standards Track                                   M. Kucherawy
ISSN: 2070-1721                                          Cloudmark, Inc.
                                                          September 2012

            Indicating Email Handling States in Trace Fields


   This document registers a trace field clause for use in indicating
   transitions between handling queues or processing states, including
   enacting inter- and intra-host message transitions.  This might
   include message quarantining, mailing list moderation, timed
   delivery, queuing for further analysis, content conversion, or other
   similar causes, as well as optionally identifying normal handling

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. Key Words .......................................................3
   3. Trace Clause ....................................................3
   4. Discussion ......................................................6
   5. Granularity .....................................................6
   6. IANA Considerations .............................................7
      6.1. MAIL Parameters Additional-registered-clauses
           Sub-Registry ...............................................7
      6.2. MAIL Parameters Registered-states Sub-Registry .............7
   7. Security Considerations .........................................9
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10
   Appendix A.  Trace Field Examples .................................11
     A.1.  Typical Delivery without Obvious Extra Handling ...........11
     A.2.  Delivery with Moderation ..................................11
   Appendix B.  Acknowledgements .....................................12

1.  Introduction

   [SMTP] defines the content of email message trace fields, commonly
   the "Received" header field.  These are typically used to record an
   audit trail of the path a message follows from origin to destination,
   with one such field added each time a message moves from one host to
   the next.

   Section 3.7.2 of that document mentions that "the most important use
   of Received: lines is for debugging mail faults [...]".

   There are some cases where there may be large time gaps between trace
   fields.  Though this might be caused by transient communication
   issues, they might also be caused by policy decisions or special
   processing regarding the content of the message, authorization of
   some identity on the message, or transitions between major software
   components.  Common examples include message quarantines (filters
   that cause a message to be held pending further evaluation or
   delivery of a message pending manual operator action), pending
   content analysis, or mailing list servers that impose moderation
   rules (mailing list owner action required regarding mail from authors
   not subscribed to those lists).

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   This document registers a new optional clause that can be used in
   trace fields to indicate that a message entered such a special
   processing queue or state for some period.  This allows inspection of
   the trace information to reveal that the cause for a time gap in
   trace fields was imposed by additional processing rather than one
   caused by transient technical difficulties.

2.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

3.  Trace Clause

   This specification defines a clause, called "state", which MAY be
   used when creating a Received header field (see Section 4.4 of
   [SMTP]) to indicate the nature of additional handling imposed on the
   relaying of a message toward its recipient(s).  It is followed by a
   single keyword that provides that detail.  A Mail Transfer Agent
   (MTA) or other handling agent that determines a message has entered a
   state other than normal queuing of messages for relaying or delivery
   MAY generate a trace field including one of these clauses.  That is,
   the presence of this clause on a trace field is an indication of the
   entry of the message into that state; a later trace field added would
   indicate its departure from that state.

   An MTA implementing this specification SHOULD add a Received field as
   described whenever:

   a.  It determines that a special handling condition will occur and
       places it into that condition; or

   b.  It determines that no special handling is required and prepares
       it for relay to the next handling agent.

   An MTA need not add a Received field indicating preparation for
   normal handoff to the next handling agent if it has already added a
   Received field for some other reason.  Trace data added by the next
   handling agent will imply the message's exit from the special
   handling condition.

   If a single MTA processes a message through multiple special handling
   conditions, it MAY add a Received field for each distinct condition.

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   For example, presume a message will be injected into MTA-1, then
   travel to MTA-3 via MTA-2, and then MTA-3 will enact final delivery.
   At MTA-2, it is determined that some action will be taken that will
   cause the message to undergo some handling change that is outside of
   typical message flow.  In this case:

   1.  MTA-1 adds a typical Received field and relays it to MTA-2.

   2.  MTA-2 determines that the atypical handling will occur and adds a
       Received field using the extension specified here.

   3.  On completion of the atypical handling, MTA-2 relays the message
       to MTA-3.

   4.  MTA-3 adds a typical Received field and enacts final delivery of
       the message.

   Appropriate use of this mechanism does not include associating meta-
   data with the message, such as categorizing the message (e.g., the
   notions of "is spam" or "was 8-bit, converted to 7-bit").  Processing
   agents also cannot reliably use this mechanism to determine anything
   about the message content, since there is no guarantee that all
   agents in the chain of handling made such annotations to allow
   correct conclusions.  The sole purpose here is to allow one to
   determine the point(s) in the chain of custody of a message at which
   the message was subjected to handling outside of normal message
   routing and queuing.

   The following state keywords are defined in this document; extensions
   may define other registered keywords (see Section 6.2):

   auth:  The message entered a queue pending authentication of some
      identifier in the message.

   content:  The message entered a queue pending content analysis, such
      as scanning for spam or viruses.

   convert:  The message entered a queue pending content conversion.

   moderation:  The message entered a hold pending mailing list
      moderator action.

   normal:  The message is not in an administrative hold and is queued
      for or is being handed off to the next handling agent (which may
      be local delivery).  This is the default interpretation when no
      "state" clause is present.

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   other:  The message entered a hold or queue for reasons not covered
      by other keywords in this list and not for transient technology

   outbound:  The message entered a queue for outbound relaying.  This
      is typically the last case added for a single host, and the next
      Received header field is expected to be added by some other host.

   quarantine:  The message entered a hold in an isolation queue pending
      operator action for local policy reasons.

   timed:  The message entered a hold in order to meet a requested
      delivery window, such as is defined in [FUTURERELEASE].

   In Section 6, the "state" clause is added to the Additional-
   Registered-Clauses IANA sub-registry.  The ABNF for this clause is:

      State = CFWS "state" FWS queue-state-keyword [ "/" value ]

      queue-state-keyword = ( reg-state-keyword / unreg-state-keyword )

      reg-state-keyword = ( "auth" / "content" / "convert" /
                            "moderation" / "normal" / "other" /
                            "outbound" / "quarantine" / "timed" /
                            additional-state-keyword )

      additional-state-keyword = token
                               ; MUST be registered; see
                               ; "IANA Considerations" below

      value = token

      unreg-state-keyword = token

   "FWS" and "CFWS" are defined in [MAIL]. "token" is defined in [MIME].

   A transfer agent making use of this extension MAY also include header
   field comments to provide additional information.

   The "value" is available for providing additional labels as
   explanation for the state transition.  Examples could include:

   o  convert/unicode2ascii

   o  moderation/not-subscribed

   o  quarantine/spam

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4.  Discussion

   Handling agents are not expected to implement or support all of
   these.  Indeed, recording trace information for all of the states
   described above could make the header of a message inordinately
   large.  Rather, an agent is encouraged to apply state annotations
   when a message enters a handling queue where a significant condition
   occurs or where significant additional processing or delay is
   possible, and especially when a handoff has occurred between two
   different, independent agents.

   For example, an MTA receiving a message, doing message
   authentication, scanning for viruses and spam, and then putting it in
   an outbound queue could add four Received header fields denoting each
   of these states.  However, where they are all done as part of a
   single system process, in a single pass, doing so would be considered
   unusual (and extremely verbose).  This method SHOULD NOT be applied
   except when doing detailed analysis of a single component to identify
   performance issues with those steps.

   Rather, an agent that wishes to make a state annotation SHOULD add
   only a single Received header field including such annotation, thus
   indicating (a) the time of completion of its handling of the message
   via the date portion of the field and (b) the final disposition of
   that message relative to that agent.  For example, an MTA receiving a
   message that performs various checks on the message before
   immediately handing it off to a Mailing List Manager (MLM) would only
   record a "normal" state, assuming it passes those checks.  The MLM
   would then evaluate the message and record its own state once it
   decides what the next step will be for the handling of that message.

5.  Granularity

   The degree of granularity -- and therefore the degree of verbosity --
   recorded through the use of this additional trace clause is likely to
   vary depending on circumstances.  It will typically be the case that
   use of this clause will be limited to "unusual" transitions, such as
   when a message requires additional scrutiny or other processing or
   needs to be quarantined.

   Somewhat greater granularity might also include transitions of
   administrative responsibility, such as between a Mail Transfer Agent
   (MTA) operator and a Mailing List Manager (MLM) operator.  This could
   be further enhanced to note some transitions that are interesting
   only when other transitions have occurred, such as noting entry to
   the outbound queue only when the message is originating from an
   "interesting" source, like an MLM, since an MLM can introduce
   significant changes to the message or delivery delay and it could be

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   useful to know when it completed its processing, as distinct from the
   subsequent processing by the originating MTA.  In circumstances
   needing very fine-grained trace information, fields might be created
   to note all of these "significant" network architecture transitions.

   One should note, however, when choosing higher levels of granularity,
   that the Received header fields present on a message could be counted
   by MTAs when trying to decide whether or not a message routing loop
   is in effect.  A message with an abundance of these might cause an
   incorrect determination that the message is in a delivery loop,
   causing it to be removed from the mail stream.  See Section 6.3 of
   [SMTP] for further discussion.

6.  IANA Considerations

6.1.  MAIL Parameters Additional-registered-clauses Sub-Registry

   This document adds the following entry to the "Additional-registered-
   clauses" sub-registry of the "MAIL Parameters" registry, created by

   Clause name:  state

   Description:  Indicates entry into a special queue state

   Syntax Summary:  state <state-name>

   Reference:  [RFC6729]

6.2.  MAIL Parameters Registered-states Sub-Registry

   The "MAIL Parameters" registry at IANA has been updated by the
   creation of the "Registered-states" sub-registry to contain valid
   state keywords for use with this specification.  Updates to this
   registry are governed by the First Come, First Served rules of [IANA]
   for new registrations.  Changes to the status of existing entries are
   limited to the original registrant or IESG approval.

   Discussion of all registry updates is encouraged via one or more IETF
   mailing lists that typically cover email-related subjects prior to
   approval of the change, as a way of documenting the work.  The list is suggested.

   Note that only registrations of queue state keywords are permitted.
   The registry is not to be used for specifying secondary information
   (i.e., the "value" part of the ABNF in Section 3).

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   Registrations are to include the following entries:

   Name:  The name of the state keyword being defined or updated, which
      conforms to the ABNF shown in Section 3.

   Description:  A brief description of the keyword's meaning.

   Specification:  The specification document that defines the queue
      state being registered, or if no stable reference exists, a more
      detailed explanation of the queue state than is in the
      "Description", sufficient to allow interoperability.

   Use:  One of "current" (the state keyword is in current use),
      "deprecated" (the state keyword is in use but not recommended for
      new implementations), or "historic" (the state keyword is no
      longer in substantial current use).

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   The initial registration set is as follows:

    | Name       | Description            | Specification   | Use     |
    | auth       | Held for message       |    [RFC6729]    | current |
    |            | authentication         |                 |         |
    | content    | Held for message       |    [RFC6729]    | current |
    |            | content analysis       |                 |         |
    | convert    | Held for message       |    [RFC6729]    | current |
    |            | content conversion     |                 |         |
    | moderation | Held for list          |    [RFC6729]    | current |
    |            | moderation             |                 |         |
    | normal     | Message is not being   |    [RFC6729]    | current |
    |            | held other than to     |                 |         |
    |            | accommodate typical    |                 |         |
    |            | relaying handling      |                 |         |
    | other      | Held for causes not    |    [RFC6729]    | current |
    |            | covered by other       |                 |         |
    |            | registered state       |                 |         |
    |            | keywords               |                 |         |
    | outbound   | Message placed in      |    [RFC6729]    | current |
    |            | outbound queue         |                 |         |
    | quarantine | Held for operator      |    [RFC6729]    | current |
    |            | action due to content  |                 |         |
    |            | analysis or local      |                 |         |
    |            | policy                 |                 |         |
    | timed      | Held to accommodate a  |    [RFC6729]    | current |
    |            | specific requested     |                 |         |
    |            | delivery window        |                 |         |

7.  Security Considerations

   The use of this trace information can reveal hints as to local policy
   that was in effect at the time of message handling.

   Further discussion about trace field security can be found in Section
   7.6 of [SMTP].

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8.  References

8.1.  Normative References

   [IANA]           Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 5226, May 2008.

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

   [MAIL]           Resnick, P., Ed., "Internet Message Format",
                    RFC 5322, October 2008.

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

   [SMTP]           Klensin, J., "Simple Mail Transfer Protocol",
                    RFC 5321, October 2008.

8.2.  Informative References

   [FUTURERELEASE]  White, G. and G. Vaudreuil, "SMTP Submission Service
                    Extension for Future Message Release", RFC 4865,
                    May 2007.

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Appendix A.  Trace Field Examples

   This section includes a sample of the new trace field clause in use.

A.1.  Typical Delivery without Obvious Extra Handling

   Typical message delivery

        Received: from
                  ( [])
              by (8.11.6/8.11.6)
                  with ESMTP id i7PK0sH7021929
                  for <>;
              Fri, Feb 15 2002 17:19:22 -0800
        Received: from
                  ( [])
              by (8.11.6/8.11.6)
                  with ESMTP id i9MKZCRd064134
                  for <>;
              Fri, Feb 15 2002 17:19:08 -0800

   Example 1: Typical message delivery with no appreciable extra
   handling; only Received header fields shown

A.2.  Delivery with Moderation

   Message delivery after moderation

        Received: from
                  ( [])
              by (8.11.6/8.11.6)
                  with ESMTP id i7PK0sH7021929
                  for <>;
              Fri, Feb 15 2002 18:33:29 -0800
        Received: from
                  ( [])
              by (8.11.6/8.11.6)
                  with ESMTP id i9MKZCRd064134
                  for <>
                  state moderation (sender not subscribed);
              Fri, Feb 15 2002 17:19:08 -0800

   Example 2: Message held for moderation; only Received header fields

   The message passed from to
   intended for a mailing list hosted at the latter.  For list
   administrative reasons, the message is held there for moderation.  It

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   is finally released over an hour later and passed to the next host.
   A comment after the state expression indicates the actual cause for
   the administrative hold.

Appendix B.  Acknowledgements

   The authors wish to acknowledge the following for their reviews and
   constructive criticisms of this proposal: Tony Finch, Ned Freed, Carl
   S. Gutenkunst, John Levine, Bill McQuillan, S. Moonesamy, Alexey
   Melnikov, Robert A. Rosenberg, Hector Santos, Rolf Sonneveld, and
   Mykyta Yevstifeyev.

Authors' Addresses

   D. Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale  94086

   Phone: +1.408.246.8253

   Murray S. Kucherawy
   Cloudmark, Inc.
   128 King St., 2nd Floor
   San Francisco, CA  94107


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