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 (C) The Internet Society (2004).
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
base64 = %x2b / %x2f / %x30-39 / %x41-5a / %x61-7a
The definition of the DIGIT token is imported from [RFC-MSGFMT].
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
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
(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.
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 ]
mtrk-certifier = base64 ; authenticator
mtrk-timeout = 1*9DIGIT ; seconds until timeout
A is stored in the originator's tracking database to validate future
tracking requests as described in [RFC-MTRK-MTQP]. B is stored in
tracking databases of compliant receiver MTAs and used to
authenticate future tracking requests.
The mtrk-timeout field indicates the number of seconds that the
client requests that this tracking information be retained on
intermediate servers, as measured from the initial receipt of the
message at that server. Servers MAY ignore this value if it violates
local policy. In particular, servers MAY silently enforce an upper
limit to how long they will retain tracking data; this limit MUST be
at least one day.
If no mtrk-timeout field is specified then the server should use a
local default. This default SHOULD be 8-10 days and MUST be at least
one day. Notwithstanding this clause, the information MUST NOT be
expired while the message remains in the queue for this server: that
is, an MTQP server MUST NOT deny knowledge of a message while that
same message sits in the MTA queue.
If the message is relayed to another compliant SMTP server, the MTA
acting as the client SHOULD pass an mtrk-timeout field equal to the
remaining life of that message tracking information. Specifically,
the tracking timeout is decremented by the number of seconds the
message has lingered at this MTA and then passed to the next MTA. If
the decremented tracking timeout is less than or equal to zero, the
entire MTRK parameter MUST NOT be passed to the next MTA;
essentially, the entire tracking path is considered to be lost at
See [RFC-DELIVERYBY] section 4 for an explanation of why a timeout is
used instead of an absolute time.
3.2. Use of ENVID
To function properly, Message Tracking requires that each message
have a unique identifier that is never reused by any other message.
For that purpose, if the MTRK parameter is given, an ENVID parameter
MUST be included, and the syntax of ENVID from RFC 3461 is extended
envid-parameter = "ENVID=" unique-envid
unique-envid = local-envid "@" fqhn
local-envid = xtext
fqhn = xtext
The unique-envid MUST be chosen in such a way that the same ENVID
will never be used by any other message sent from this system or any
other system. In most cases, this means setting fqhn to be the fully
qualified host name of the system generating this ENVID, and local-
envid to an identifier that is never re-used by that host.
In some cases, the total length of (local-envid + fqhn + 1) (for the
`@' sign) may exceed the total acceptable length of ENVID (100). In
this case, the fqhn SHOULD be replaced by the SHA1(fqhn) encoded into
BASE64. After encoding, the 160 bit SHA-1 will be a 27 octet string,
which limits local-envid to 72 octets. Implementors are encouraged
to use an algorithm for the local-envid that is reasonably unique.
For example, sequential integers have a high probability of
intersecting with sequential integers generated by a different host,
but a SHA-1 of the current time of day concatenated with the host's
IP address and a random number are unlikely to intersect with the
same algorithm generated by a different host.
Any resubmissions of this message into the message transmission
system MUST assign a new ENVID. In this context, "resubmission"
includes forwarding or resending a message from a user agent, but
does not include MTA-level aliasing or forwarding where the message
does not leave and re-enter the message transmission system.
3.3. Forwarding Tracking Certifiers
MTAs SHOULD forward unexpired tracking certifiers to compliant
mailers as the mail is transferred during regular hop-to-hop
transfers. If the "downstream" MTA is not MTRK-compliant, then the
MTRK= parameter MUST be deleted. If the downstream MTA is DSN-
compliant, then the ENVID and ORCPT parameters MUST NOT be deleted.
If aliasing, forwarding, or other redirection of a recipient occurs,
and the result of the redirection is exactly one recipient, then the
MTA SHOULD treat this as an ordinary hop-to-hop transfer and forward
the MTRK=, ENVID=, and ORCPT= values; these values MUST NOT be
modified except for decrementing the mtrk-timeout field of the MTRK=
value, which MUST be modified as described in section 4.1 above.
MTAs MUST NOT copy MTRK certifiers when a recipient is aliased,
forwarded, or otherwise redirected and the redirection results in
more than one recipient. However, an MTA MAY designate one of the
multiple recipients as the "primary" recipient to which tracking
requests shall be forwarded; other addresses MUST NOT receive
tracking certifiers. MTAs MUST NOT forward MTRK certifiers when
doing mailing list expansion.
4. Security Considerations
4.1. Denial of service
An attacker could attempt to flood the database of a server by
submitting large numbers of small, tracked messages. In this case, a
site may elect to lower its maximum retention period retroactively.
The mtrk-authenticator value ("A") must be hard to predict and not
The originating client must take reasonable precautions to protect
the secret. For example, if the secret is stored in a message store
(e.g., a "Sent" folder), the client must make sure the secret isn't
accessible by attackers, particularly on a shared store.
Many site administrators believe that concealing names and topologies
of internal systems and networks is an important security feature.
MTAs need to balance such desires with the need to provide adequate
In some cases site administrators may want to treat delivery to an
alias as final delivery in order to separate roles from individuals.
For example, sites implementing "postmaster" or "webmaster" as
aliases may not wish to expose the identity of those individuals by
permitting tracking through those aliases. In other cases, providing
the tracking information for an alias is important, such as when the
alias points to the user's preferred public address.
Therefore, implementors are encouraged to provide mechanisms by which
site administrators can choose between these alternatives.
5. IANA Considerations
IANA has registered the SMTP extension defined in section 3.
Several individuals have commented on and enhanced this document,
including Philip Hazel, Alexey Melnikov, Lyndon Nerenberg, Chris
Newman, and Gregory Neil Shapiro.
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-ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF
for Syntax Specifications: ABNF", RFC 2234,
[RFC-ESMTP] Klensin, J., Freed, N., Rose, M., Stefferud, E.,
and D. Crocker, "SMTP Service Extensions", STD 10,
RFC 1869, November 1995.
[RFC-KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
[RFC-MIME] Freed, N. and N. Borenstein, "Multipurpose
Internet Mail Extensions (MIME) Part One: Format
of Internet Message Bodies", RFC 2045, November
[NIST-SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard"
National Institute of Standards and Technology,
U.S. Department of Commerce, May 1994.
[RFC-SMTP] Klensin, J., Ed., "Simple Mail Transfer Protocol",
RFC 2821, April 2001.
[RFC-MSGFMT] Resnick, P., Ed., "Internet Message Format", RFC
2822, April 2001.
7.2. Informational References
[RFC-DELIVERYBY] Newman, D., "Deliver By SMTP Service Extension",
RFC 2852, June 2000.
[RFC-DSN-SMTP] Moore, K., "Simple Mail Transfer Protocol (SMTP)
Service Extension for Delivery Status
Notifications (DSNs)", RFC 3461, January 2003.
[RFC-MDN] Hansen, T. and G. Vaudreuil, Eds., "Message
Disposition Notification", RFC 3798, May 2004.
[RFC-RANDOM] Eastlake, D., Crocker, S., and J. Schiller,
"Randomness Recommendations for Security", RFC
1750, December 1994.
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
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
Funding for the RFC Editor function is currently provided by the