<- RFC Index (5001..5100)
(also BCP 134)
Updated by RFC 8314
Network Working Group C. Hutzler
Request for Comments: 5068
BCP: 134 D. Crocker
Category: Best Current Practice Brandenburg InternetWorking
P. Resnick
QUALCOMM Incorporated
E. Allman
Sendmail, Inc.
T. Finch
University of Cambridge Computing Service
November 2007
Email Submission Operations: Access and Accountability Requirements
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Abstract
Email has become a popular distribution service for a variety of
socially unacceptable, mass-effect purposes. The most obvious ones
include spam and worms. This note recommends conventions for the
operation of email submission and transport services between
independent operators, such as enterprises and Internet Service
Providers. Its goal is to improve lines of accountability for
controlling abusive uses of the Internet mail service. To this end,
this document offers recommendations for constructive operational
policies between independent operators of email submission and
transmission services.
Email authentication technologies are aimed at providing assurances
and traceability between internetworked networks. In many email
services, the weakest link in the chain of assurances is initial
submission of a message. This document offers recommendations for
constructive operational policies for this first step of email
sending, the submission (or posting) of email into the transmission
network. Relaying and delivery entail policies that occur subsequent
to submission and are outside the scope of this document.
Hutzler, et al. Best Current Practice [Page 1]
RFC 5068 Email Submission November 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Submission, Relaying, Delivery . . . . . . . . . . . . . . . . 4
3.1. Best Practices for Submission Operation . . . . . . . . . 5
3.2. Transitioning to Submission Port . . . . . . . . . . . . . 6
4. External Submission . . . . . . . . . . . . . . . . . . . . . 6
4.1. Best Practices for Support of External Submissions . . . . 7
5. Message Submission Authentication/Authorization
Technologies . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 10
Hutzler, et al. Best Current Practice [Page 2]
RFC 5068 Email Submission November 2007 1. Introduction The very characteristics that make email such a convenient communications medium -- its near ubiquity, rapid delivery, low cost, and support for exchanges without prior arrangement -- have made it a fertile ground for the distribution of unwanted or malicious content. Spam, fraud, and worms have become a serious problem, threatening the viability of email and costing end users and providers millions of dollars in damages and lost productivity. In recent years, independent operators including enterprises and ISPs have turned to a number of different technologies and procedures, in an attempt to combat these problems. The results have been mixed, at best. En route to its final destination, email will often travel between multiple independent providers of email transmission services. These services will generally have no prior arrangement with one another and may employ different rules on the transmission. It is therefore difficult both to debug problems that occur in mail transmission and to assign accountability if undesired or malicious mail is injected into the Internet mail infrastructure. Many email authentication technologies exist. They provide some accountability and traceability between disparate networks. This document aims to build upon the availability of these technologies by exploring best practices for authenticating and authorizing the first step of an email's delivery, from a Mail User Agent (MUA) to a Mail Submission Agent (MSA), known as submission. Without strong practices on email submission, the use of authentication technologies elsewhere in the service provides limited benefit. This document specifies operational policies to be used for the first step of email sending, the submission -- or posting from an MUA to an MSA as defined below -- of email into the transmission service. These policies will permit continued, smooth operation of Internet email, with controls added to improve accountability. Relaying and delivering employ policies that occur after submission and are outside the scope of this document. The policies listed here are appropriate for operators of all sizes of networks and may be implemented by operators independently, without regard for whether the other side of an email exchange has implemented them. It is important to note that the adoption of these policies alone will not solve the problems of spam and other undesirable email. However, these policies provide a useful step in clarifying lines of accountability and interoperability between operators. This helps raise the bar against abusers and provides a foundation for additional tools to preserve the utility of the Internet email infrastructure. Hutzler, et al. Best Current Practice [Page 3]
RFC 5068 Email Submission November 2007
NOTE: This document does not delve into other anti-spam operational
issues such as standards for rejection of email. The authors note
that this could be a valuable effort to undertake and encourage
its pursuit.
2. Terminology
The Internet email architecture distinguishes four message-handling
components:
o Mail User Agents (MUAs)
o Mail Submission Agents (MSAs)
o Mail Transfer Agents (MTAs)
o Mail Delivery Agents (MDAs)
At the origination end, an MUA works on behalf of end users to create
a message and perform initial "submission" into the transmission
infrastructure, via an MSA. An MSA accepts the message submission,
performs any necessary preprocessing on the message, and relays the
message to an MTA for transmission. MTAs 'relay' messages to other
MTAs, in a sequence reaching a destination MDA that, in turn,
'delivers' the email to the recipient's inbox. The inbox is part of
the recipient-side MUA that works on behalf of the end user to
process received mail.
These architectural components are often compressed, such as having
the same software do MSA, MTA and MDA functions. However the
requirements for each of these components of the architecture are
becoming more extensive, so that their software and even physical
platform separation is increasingly common.
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 [RFC2119].
3. Submission, Relaying, Delivery
Originally the MSA, MTA, and MDA architectural components were
considered to be a single unit. This was reflected in the practice
of having MSA, MTA, and MDA transfers all be performed with SMTP
[RFC2821] [RFC821], over TCP port 25. Internet mail permits email
to be exchanged without prior arrangement and without sender
authentication. That is, the confirmed identity of the originator of
the message is not necessarily known by the relaying MTAs or the MDA.
Hutzler, et al. Best Current Practice [Page 4]
RFC 5068 Email Submission November 2007
It is important to distinguish MUA-to-MSA email submission, versus
MTA relaying, versus the final MTA-to-MDA transition. Submission
typically does entail a pre-established relationship between the user
of the client and operator of the server; equally, the MDA is
performing final delivery and can determine that it has an existing
relationship with the recipient. That is, MSAs and MDAs can take
advantage of having prior relationships with users in order to
constrain their transfer activities.
Specifically, an MSA can choose to reject all postings from MUAs for
which it has no existing relationship. Similarly, an MDA can choose
to reject all mail to recipients for which it has no arrangement to
perform delivery. Indeed, both of these policies are already in
common practice.
3.1. Best Practices for Submission Operation
Submission Port Availability:
If external submissions are supported -- that is, from outside a
site's administrative domain -- then the domain's MSAs MUST
support the SUBMISSION port 587 [RFC4409]. Operators MAY
standardize on the SUBMISSION port for both external AND LOCAL
users; this can significantly simplify submission operations.
Submission Port Use:
MUAs SHOULD use the SUBMISSION port for message submission.
Submission Authentication:
MSAs MUST perform authentication on the identity asserted during
all mail transactions on the SUBMISSION port, even for a message
having a RCPT TO address that would not cause the message to be
relayed outside of the local administrative domain.
Submission Authorization:
An operator of an MSA MUST ensure that the authenticated identity
is authorized to submit email, based on an existing relationship
between the submitting entity and the operator. This requirement
applies to all mail submission mechanisms (MUA to MSA).
Submission Accountability after Submission:
For a reasonable period of time after submission, the message
SHOULD be traceable by the MSA operator to the authenticated
identity of the user who sent the message. Such tracing MAY be
Hutzler, et al. Best Current Practice [Page 5]
RFC 5068 Email Submission November 2007
based on transactional identifiers stored in the headers (received
lines, etc.) or other fields in the message, on audit data stored
elsewhere, or on any other mechanism that supports sufficient
post-submission accountability. The specific length of time,
after message submission, that traceability is supported is not
specified here. However, issues regarding transit often occur as
much as one week after submission.
Note that [RFC3848] defines a means of recording submission-time
information in Received header fields. This information can help
receive-side analysis software establish a sending MSA's
accountability and then make decisions about processing the
message.
3.2. Transitioning to Submission Port
In order to promote transition of initial message submission from
port 25 to port 587, MSAs MUST listen on port 587 by default and
SHOULD have the ability to listen on other ports. MSAs MUST require
authentication on port 587 and SHOULD require authentication on any
other port used for submission. MSAs MAY also listen on other ports.
Regardless of the ports on which messages are accepted, MSAs MUST NOT
permit relaying of unauthenticated messages to other domains. That
is, they must not be open relays.
As a default, MUAs SHOULD attempt to find the best possible
submission port from a list of alternatives. The SUBMISSION port 587
SHOULD be placed first in the list. Since most MUAs available today
do not permit falling back to alternate ports, sites SHOULD pre-
configure or encourage their users to connect on the SUBMISSION port
587, assuming that site supports that port.
4. External Submission
An MUA might need to submit mail across the Internet, rather than to
a local MSA, in order to obtain particular services from its home
site. Examples include active privacy protection against third-party
content monitoring, timely processing, and being subject to the most
appropriate authentication and accountability protocols. Further,
the privacy requirement might reasonably include protection against
monitoring by the operator of the MUA's access network. This
requirement creates a challenge for the provider operating the IP
network through which the MUA gains access. It makes that provider
an involuntary recruit to the task of solving mass-effect email
problems: When the MUA participates in a problem that affects large
numbers of Internet users, the provider is expected to effect
remedies and is often expected to prevent such occurrences.
Hutzler, et al. Best Current Practice [Page 6]
RFC 5068 Email Submission November 2007
A proactive technique used by some providers is to block all use of
port 25 SMTP for mail that is being sent outbound, or to
automatically redirect this traffic through a local SMTP proxy,
except for hosts that are explicitly authorized. This can be
problematic for some users, notably legitimate mobile users
attempting to use their "home" MSA, even though those users might
already employ legitimate, port 25-based authentication.
This document offers no recommendation concerning the blocking of
SMTP port 25 or similar practices for controlling abuse of the
standard anonymous mail transfer port. Rather, it pursues the
mutually constructive benefit of using the official SUBMISSION port
587 [RFC4409].
NOTE: Many established practices for controlling abuse of port 25,
for mail that is being sent outbound, currently do exist. These
include the proxy of SMTP traffic to local hosts for screening,
combined with various forms of rate limits. The authors suggest
that a separate document on this topic would benefit the email
operations community.
4.1. Best Practices for Support of External Submissions
Open Submission Port:
Access Providers MUST NOT block users from accessing the external
Internet using the SUBMISSION port 587 [RFC4409].
Traffic Identification -- External Posting (MSA) Versus Relaying
(MX):
When receiving email from outside their local operational
environment, email service providers MUST distinguish between
unauthenticated email addressed to local domains (MX traffic)
versus submission-related authenticated email that can be
addressed anywhere (MSA traffic). This allows the MTA to restrict
relaying operations, and thereby prevent "open" relays. Note that
there are situations where this may not apply, such as secondary
MXs and related implementations internal to an operator's network
and within their control.
Figure 1 depicts a local user (MUA.l) submitting a message to an MSA
(MSA). It also shows a remote user (MUA.r), such as might be in a
coffee shop offering "hotspot" wireless access, submitting a message
to their "home" MSA via an authenticated port 587 transaction. The
figure shows the alternative of using port 587 or port 25 within the
MSA's network. This document makes no recommendations about the use
Hutzler, et al. Best Current Practice [Page 7]
RFC 5068 Email Submission November 2007
of port 25 for submission. The diagram merely seeks to note that it
is in common use and to acknowledge that port 25 can be used with
sufficient accountability within an organization's network.
HOME NETWORK DESTINATION
+-------+
| MUA.l |
+---+---+
port | port port port
587/25 V 25 25 -------- 25
+-----+ +-----+ ****** / \ ****** +-----+ +-----+
| MSA |->| MTA |->* AP *->| |->* AP *->| MTA |->| MDA |
+--^--+ +-----+ ****** | INTERNET | ****** +-----+ +-----+
| | |
+-------<--------------|----+ |
\ | /
---^----
|
******
AP = Access Provider * AP *
******
| port 587
+---+----+
| MUA.r |
+--------+
HOTSPOT
Figure 1: Example of Port 587 Usage via Internet
5. Message Submission Authentication/Authorization Technologies
There are many competent technologies and standards for
authenticating message submissions. Two component mechanisms that
have been standardized include SMTP AUTH [RFC4954] and TLS [RFC3207].
Depending upon the environment, different mechanisms can be more or
less effective and convenient. Mechanisms might also have to be used
in combination with each other to make a secure system.
Organizations SHOULD choose the most secure approaches that are
practical.
This document does not provide recommendations on specific security
implementations. It simply provides a warning that transmitting user
credentials in clear text over insecure networks SHOULD be avoided in
all scenarios as this could allow attackers to listen for this
traffic and steal account data. In these cases, it is strongly
suggested that an appropriate security technology MUST be used.
Hutzler, et al. Best Current Practice [Page 8]
RFC 5068 Email Submission November 2007
6. Security Considerations
Email transfer between independent administrations can be the source
of large volumes of unwanted email and email containing malicious
content designed to attack the recipient's system. This document
addresses the requirements and procedures to permit such exchanges
while reducing the likelihood that malicious mail will be
transmitted.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
7.2. Informative References
[RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3848] Newman, C., "ESMTP and LMTP Transmission Types
Registration", RFC 3848, July 2005.
[RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954, July 2007.
Hutzler, et al. Best Current Practice [Page 9]
RFC 5068 Email Submission November 2007 Appendix A. Acknowledgments These recommendations were first formulated during informal discussions among members of Anti-Spam Technical Alliance (ASTA) and some participants from the Internet Research Task Force's Anti-Spam Research Group (ASRG). Later reviews and suggestions were provided by: M. Allman, L.H. Aestrand, N. Borenstein, S. Bortzmeyer, K. Chon, R. Clayton, B. Cole, W. Dnes, V. Duchovni, E. Edelstein, F. Ellermann, M. Elvey, J.D. Falk, N. Freed, J. Glube, A. Herzberg, J. Klensin, J. Levine, S. Moonesamy, K. Moore, R. Nelson, C. Newman, C. O'Malley, S. Ramasubramanian, R. Rognlie, J. St. Sauver, W. Schlitt, B. Shein, B. Sullivan. Authors' Addresses Carl Hutzler 2512 Freetown Drive Reston, VA 20191 Phone: 703-915-6862 EMail: cdhutzler@aol.com URI: http://carlhutzler.com/blog/ Dave Crocker Brandenburg InternetWorking 675 Spruce Dr. Sunnyvale, CA 94086 USA Phone: +1.408.246.8253 EMail: dcrocker@bbiw.net URI: http://bbiw.net Peter Resnick QUALCOMM Incorporated 5775 Morehouse Drive San Diego, CA 92121-1714 USA Phone: +1 858 651 4478 EMail: presnick@qualcomm.com URI: http://www.qualcomm.com/~presnick/ Hutzler, et al. Best Current Practice [Page 10]
RFC 5068 Email Submission November 2007 Eric Allman Sendmail, Inc. 6745 Christie Ave., Suite 350 Emeryville, CA USA Phone: +1 510 594 5501 EMail: eric+ietf-smtp@sendmail.org Tony Finch University of Cambridge Computing Service New Museums Site Pembroke Street Cambridge CB2 3QH ENGLAND Phone: +44 797 040 1426 EMail: dot@dotat.at URI: http://dotat.at/ Hutzler, et al. Best Current Practice [Page 11]
RFC 5068 Email Submission November 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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, THE IETF TRUST 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. Intellectual Property 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 http://www.ietf.org/ipr. 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-ipr@ietf.org. Hutzler, et al. Best Current Practice [Page 12]