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.
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].
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.
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.
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.
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.
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.
[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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