Network Working Group C. Hutzler
Request for Comments: 5068
BCP: 134 D. Crocker
Category: Best Current Practice Brandenburg InternetWorking
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.
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
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.
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
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
The Internet email architecture distinguishes four message-handling
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] [RFC0821], 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.
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
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.
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.
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
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
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.
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
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
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
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
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 |
Figure 1: Example of Port 587 Usage via Internet5. 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
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.
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
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,
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
7.2. Informative References
[RFC0821] 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.
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.
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