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SMTP                                                          C. Hutzler
Intended status: Best Current                                 D. Crocker
Practice                                     Brandenburg InternetWorking
Expires: November 19, 2007                                    P. Resnick
                                                   QUALCOMM Incorporated
                                                              R. Sanders

                                                               E. Allman
                                                          Sendmail, Inc.
                                                            May 18, 2007

              Email Submission: Access and Accountability

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   Email has become a popular distribution service for a variety of

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   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.  Consequently
   the document offers recommendations for constructive operational
   policies between independent operators of email transmission

   With the recent advent of email authentication technologies aimed at
   providing assurances and traceability between internetworked
   networks, the authors recognized that the initial submission of a
   message became the weakest link.  Consequently, the document offers
   recommendations for constructive operational policies for the 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

   The document seeks BCP status.  Comments and discussion of this
   document should be addressed to the ietf-smtp@imc.org mailing list.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Submission, Relaying, Delivery . . . . . . . . . . . . . . . .  5
     3.1.  Best Practices for Submission Operation  . . . . . . . . .  6
     3.2.  Transitioning to Submission Port . . . . . . . . . . . . .  7
   4.  External Submission  . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Best Practices for Support of External Submissions . . . .  8
   5.  Message Submission Authentication/Authorization
       Technologies . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  References -- Normative  . . . . . . . . . . . . . . . . . 10
     7.2.  References -- Informative  . . . . . . . . . . . . . . . . 10
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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1.  Introduction

   The very characteristics that make email such a convenient
   communications medium -- its near ubiquity, rapid delivery and low
   cost -- 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,
   with varying effect and with vastly different impacts on users and on
   the Internet mail infrastructure.

   Email will often travel between multiple independent providers of
   email transmission services, en route to its final destination.  They
   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.

   A wide variety of email authentication technologies has been
   developed, and more are under development.  They provide some
   accountability and traceability between disparate networks.  This
   document aims to build on these technologies by exploring best
   practices for authenticating and authorizing the first step of an
   email's delivery from MUA to MSA, otherwise known as submission.
   Without strong practices on email submission, the authentication
   technologies provide 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 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 they 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

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   This document does not delve into other anti-spam operational issues
   such as standards for rejection of email.  The authors note that this
   would be a very valuable effort to undertake and suggest that
   additional work under another BCP document should be embarked upon.

2.  Terminology

   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

   Note: The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   this document are to be interpreted as described in [RFC2119].

3.  Submission, Relaying, Delivery

   The MSA, MTA and MDA functions used to be considered as the same set
   of functions.  This has been reflected in the history of Internet
   mail by having MSA, MTA and MDA transfers all be performed with SMTP
   [RFC2821] [RFC0821], over TCP Port 25.  Internet mail permits email

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   to be exchanged with no prior arrangement.  Hence Port 25 exchanges
   occur 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 transmission, prior to MDA-
   to-MUA delivery.  Submission typically does entail a pre-established
   relationship between the user of the client and operator of the
   server; equally, the MDA can determine that it will be affecting
   final delivery and 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

   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 that MDA 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].  It is also
         suggested that operators standardize on the SUBMISSION port for
         both external AND LOCAL users for simplicity.

      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

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      Submission Authorization:

         Operators of MSAs MUST perform authorization of the
         authenticated identity, for the operations performed during
         mail submission and based on an existing relationship with the
         submitting entity.  This requirement applies to all mail
         submission mechanisms (MUA to MSA).

      Submission Accountability after Submission:

         Once a message has been submitted, the message SHOULD be later
         traceable by the MSA operator to the authenticated identity of
         the user who sent the message for a reasonable period of time.
         Such tracing MAY be based on transactional identifiers stored
         in the headers (received lines, etc) or other fields in the
         message.  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.

3.2.  Transitioning to Submission Port

   In order to promote transition of initial message submission from
   port 25 to port 587, MSAs SHOULD listen on both ports.  MSAs MUST
   require authentication on port 587 and SHOULD require authentication
   on port 25.  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 (i.e., they must not be
   open relays).

   As delivered from the factory, MUAs SHOULD attempt to find the best
   possible submission port from a list of alternatives.  That list
   SHOULD include the SUBMISSION port 587 as well as port 25.  The
   ordering of that list SHOULD try the SUBMISSION port 587 before
   trying port 25, and MAY try other ports before, between, or after
   those two ports.  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, desiring special services, may need to submit mail across the

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   Internet, rather than to a local MSA, in order to obtain particular
   services.  Examples include active privacy protection against third-
   party content monitoring and timely processing.  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 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 and 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: However the authors wish to note that many established
   practices for controlling abuse of port25, for mail that is being
   sent outbound, currently exist.  These include the proxy of smtp
   traffic to local hosts for screening combined with various forms of
   rate limits.  The authors suggest that this topic should be addressed
   in a separate BCP that would benefit the operational communities.

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 Versus Relaying:

         For email being received from outside their local operational
         environment, email service providers MUST distinguish between

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         mail that will be delivered inside that environment, versus
         mail that is to be relayed back out to the internet.  This
         allows the MTA to restrict this operation, preventing the
         problem embodied by "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.

      Delivery Authorization:

         MDAs MUST NOT accept mail to recipients for which that MDA has
         no arrangement to perform delivery.

   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.

                 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 Internet

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5.  Message Submission Authentication/Authorization Technologies

   There are many competent technologies and standards for
   authenticating message submissions.  Two mechanisms that have been
   standardized include SMTP AUTH [RFC2554] and TLS [RFC3207].
   Depending upon the environment, different mechanisms can be more or
   less effective and convenient.  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.

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.  References

7.1.  References -- Normative

   [RFC0821]  Postel, J., "Simple Mail Transfer Protocol", STD 10,
              RFC 821, August 1982.

   [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.  References -- Informative

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2554]  Myers, J., "SMTP Service Extension for Authentication",
              March .

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   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", February 2002.

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).

Authors' Addresses

   C. Hutzler
   2512 Freetown Drive
   Reston, VA  20191

   Phone: 703-915-6862
   Email: cdhutzler@aol.com
   URI:   http://carlhutzler.com/blog/

   D. Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale, CA  94086

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net
   URI:   http://bbiw.net

   P. Resnick
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121-1714

   Phone: +1 858 651 4478
   Email: presnick@qualcomm.com
   URI:   http://www.qualcomm.com/~presnick/

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   R. Sanders
   Atlanta, GA


   E. Allman
   Sendmail, Inc.
   Emeryville, CA

   Phone: +1 510 594 5501
   Email: eric+ietf-smtp@sendmail.org

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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

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