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Network Working Group                                           C. Hutzler
Internet-Draft                                              America Online
Expires:   March 24, 2006                                       D. Crocker
                                               Brandenburg InternetWorking
                                                                P. Resnick
                                                     QUALCOMM Incorporated
                                                                R. Sanders
                                                           Earthlink, Inc.
                                                                 E. Allman
                                                            Sendmail, Inc.
                                                           24 October 2005

             Email Submission: Access and Accountability

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions of
   Section 3 of RFC 3978. By submitting this Internet-Draft, each author
   represents that any applicable patent or other IPR claims of which he or
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   This Internet-Draft will expire on October 6, 2006.

   Copyright Notice: Copyright (C) The Internet Society (2005).

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

   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 document.

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

   Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Submission, Relaying, Delivery . . . . . . . . . . . . . . .  6
   4.  External Submission  . . . . . . . . . . . . . . . . . . . .  8
   5.  Message Submission Authentication Technologies . . . . . . .  10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . .  10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1   References -- Normative  . . . . . . . . . . . . . . . .  11
     7.2   References -- Informative  . . . . . . . . . . . . . . .  11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  12
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  13
       Intellectual Property and Copyright Statements . . . . . . .  14

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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.

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   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

   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

   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

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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",
   "OPTIONAL" in this document are to be interpreted as described in

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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 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 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 that MDA has no
   arrangement to perform delivery.  Indeed, both of these policies
   are already in common practice.

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   Submission Port Availability:
          MSAs MUST support the SUBMISSION port 587 [RFC2476] for MUA
          access from outside the MSA’s local environment. 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 environment.

   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.

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4.  External Submission

   An MUA, desiring special services, may need to submit mail across
   the 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
   outbound Port 25 SMTP traffic 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 [RFC2476].

   Note: However the authors wish to note that many established
   practices for controlling abuse of outbound port25 traffic exist
   including 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.

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   Open Submission Port:
          Access Providers MUST NOT block users from accessing the
          external Internet using the SUBMISSION port 587 [RFC2476].

   Traffic Identification -- External Posting Versus Relaying:
          For email being received from outside their local
          operational environment, email service providers MUST
          distinguish between 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

             HOME  NETWORK                                DESTINATION
                        port             --------  port
    +-------+    +-----+ 25 +-----+     /        \  25 +-----+    +-----+
    | MUA.l |--->| MSA |--->| MTA |--->|          |--->| MTA |--->| MDA |
    +-------+    +--^--+    +-----+    | INTERNET |    +-----+    +-----+
             port   |                  |          |
            25/587  +-----------<------|----+     |
                                        \   |    /
                                            | 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 approach that is 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 transmitted.

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   7.  References

   7.1  References -- Normative

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

      [RFC2476]  Gellens, R. and J. Klensin, "Message Submission",
                 RFC 2476, December 1998.

      [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
                 April 2001.

   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",
                 RFC 2554, March 1999.

      [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
                 Transport Layer Security", RFC 3207, February 2002.

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Authors' Addresses

   Carl Hutzler
   America Online
   12100 Sunrise Valley Drive
   Reston, VA  20191

   Phone:  +1 703 265 5521
   Email:  cdhutzler@aol.com

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Drive
   Sunnyvale, CA  94086

   Phone:  +1.408.246.8253
   Email:  dcrocker@bbiw.net

   Peter W. 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/

   Robert Sanders
   Earthlink, Inc.
   1375 Peachtree Street
   Atlanta, GA  30309

   Phone:  +1 404 748 7021
   Email:  sandersr@corp.earthlink.net
   URI:    http://home.mindspring.com/~rsanders/

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   Eric Allman
   Sendmail, Inc.
   Emeryville, CA  94608

   Phone:  +1 510 594 5501
   Email:  eric@sendmail.com

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).

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Copyright Statement
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