RFC 3886

An Extensible Message Format for Message Tracking Responses

📅 published September 2004, from ietf

RFC 3886: An Extensible Message Format for Message Tracking Responses 💌

  • Series and number: Network Working Group, Request for Comments: 3886
  • Title: An Extensible Message Format for Message Tracking Responses
  • Author: E. Allman, Sendmail, Inc. ✍️
  • Date: September 2004 📅
  • Category: Standards Track 🛤️
  • Updates: 3463 🔧

Bestie, this one is all about how an email server tells you what happened to a message you're tracking down. It updates RFC 3463 and adds a brand new MIME type to the mix 💅

Status of This Memo 📜

This document specifies an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🗣️ For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🌸

Copyright (C) The Internet Society (2004).

Abstract ✨

Okay so, Message Tracking is expected to be used to figure out the status of undelivered e-mail when somebody asks for it 📬 It works alongside Delivery Status Notifications (DSN) and Message Disposition Notifications (MDN). Generally, a tracking request only gets sent once a DSN or MDN hasn't shown up within a reasonable timeout period. Iconic behaviour, honestly 🥹

This memo defines a MIME content-type for message tracking status, kind of in the same spirit as RFC 3464, "An Extensible Message Format for Delivery Status Notifications". It gets issued in response to a request described in "Message Tracking Query Protocol". This memo only defines the format of the status information 🎀 An extension to SMTP that labels messages for further tracking and requests tracking status is defined in a separate memo, so that's a whole other story 🧚

1. Introduction 🌷

Message Tracking is expected to be used to determine the status of undelivered e-mail upon request. Tracking is used together 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] give you 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 brought 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]. Yes, these are the official BCP 14 vibes, so read them like the rules they are 🫶

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", and 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 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 🤓

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. They're 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 💫

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.

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. Slay, this is the new code everyone needs to know! 🌈💅

3.3.5. Remote-MTA field ✨

okay bestie, the Remote-MTA field is defined the same way as section Reference 2.3.5 of [RFC-DSN-STAT] 💅 It MUST NOT be included if no delivery attempts have been made, or if the Action field has the value "opaque" 🙈 If delivery goes to some agent other than an MTA (like a Local Delivery Agent), then this field MAY be included, and it gives the name of the host where 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] 🫶 It is REQUIRED if any delivery attempt has been made and the Action field does not have the value "opaque". When it's there, it tells you when the message was last tried for delivery to another MTA or other Delivery Agent ⏰ If no delivery attempts have been made, this field MUST NOT be included 🚫

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 described 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 is the lowkey special case 👀 That means an LDA speaking LMTP [RFC-LMTP] that supports the MTRK extension. In that situation, the LDA SHOULD be passed the tracking request ✨ Then the Action field for the MTA->LDA exchange looks the same as a transfer to a compliant MTA. In other words, a "transferred" tracking status gets issued 🦋

4. Security Considerations 🔐

4.1. Forgery 🎭

ngl this part is kinda scary 😭 Malicious servers may try to mess with message tracking and send back false information. That could lead to misdirection or misinterpretation of the results 💀

4.2. Confidentiality 🤫

confidentiality is the other big security vibe 💜 There can be cases where a message recipient is autoforwarding messages but doesn't want to reveal the address the messages are forwarded to. That desire for privacy will probably get even stronger as "wireless mailboxes", like pagers, become more popular as autoforward addresses 📟

MTA authors are encouraged to provide a mechanism that lets the end user keep a forwarding address confidential 🔒 Depending on how much confidentiality is needed, and on the nature of the environment the message is being forwarded into, this could be done by one or more of these:

  • (a) respond with a "relayed" tracking status when a message is forwarded to a confidential forwarding address, and disable further message tracking requests.
  • (b) declare the message to be delivered, issue a "delivered" tracking status, re-send the message to the confidential forwarding address, and disable 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. no peeking at the list members, periodt 🙅‍♀️

5. IANA Considerations 📚

IANA has registered the SMTP extension defined in section 3 🎉

6. Acknowledgements 🙌

several individuals have commented on and enhanced this document, and they deserve all the flowers 💐 They are 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.
  • [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.
  • [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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