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Network Working Group                                         C. Hutzler
Internet-Draft                                                       AOL
Expires: March 15, 2004                                       D. Crocker
                                             Brandenburg Internetworking
                                                              P. Resnick
                                                   QUALCOMM Incorporated
                                                              R. Sanders
                                                         Earthlink, Inc.
                                                               E. Allman
                                                          Sendmail, Inc.
                                                          March 26, 2004
                                              Updated September 15, 2004

             Email Submission Between Independent Networks

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This Internet-Draft will expire on March 15, 2004

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   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

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   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

   The document will seek BCP status. Comments and discussion of this
   document should be addressed to the ietf-smtp@imc.org mailing list
   or the Ant-Spam Research Group (ASRG).

Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2. Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3. Submission, Relaying, Delivery . . . . . . . . . . . . . . . .   5
   4. External Submission  . . . . . . . . . . . . . . . . . . . . .   6
   5. Message Submission Authentication Technologies . . . . . . . .   8
   6. Security Considerations  . . . . . . . . . . . . . . . . . . .   9
   A. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  10
      Normative References . . . . . . . . . . . . . . . . . . . . .  11
      Informative References . . . . . . . . . . . . . . . . . . . .  12
      Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  12
      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 processes, in
   an attempt to combat these problems, with varying effect and with
   vastly different impacts on users and on the Internet mail

   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.

   This document suggests operational policies that independent
   operators of email transmission services may adopt, to assist in
   providing continued, smooth operation of Internet email, but with
   controls in place to improve accountability. These policies 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 will help
   raise the bar for abusers, and will pave the way for additional tools
   to preserve the utility of the Internet email infrastructure.

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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 performs initial "submission" into the transmission
   infrastructure, via an MSA.  An MSA accepts the message submission,
   performs any necessary pre- processing 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 separation is increasingly

      Note: The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
      "OPTIONAL" in this document are to be interpreted as described in
      RFC 2119 [3].

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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
   [2], 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 sender 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 relationship
   between client and server; equally, the MDA can determine that it
   will be effecting 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.

   Best practices are:

   o  Operators of MSAs MUST perform authentication during mail
      submission, based on an existing relationship with the submitting
      entity. This requirement applies to all mail submission

   o  For email being received from outside their local operational
      environment, email service operators MUST distinguish between mail
      that will be delivered inside that environment, from mail that is
      to be relayed back out to the Internet.  This prevents the problem
      embodied by "open" relays.

   o  Mail coming from outside an email operator's local environment,
      and having a RCPT-TO address that resolves to a destination
      outside the local environment, MUST be treated as mail submission,
      rather than mail relaying.

   o  MDAs SHALL NOT accept mail to recipients for which that MDA has no
arrangement to perform

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4. External Submission

   An MUA, such as one desiring enforced privacy, may need to submit
   mail across the Internet, rather than to a local MSA. This
   requirement creates a challenge for the provider operating the
   network that hosts the MUA.  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 operator 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 force 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.

   Rather, it pursues the constructive benefit of using the official
   SUBMISSION Port 587 [1].

   Best practices are:

   o  MSAs MUST support the SUBMISSION port, for MUAs accessing from
      outside the MSA's local environment.

   o  MSAs MUST perform authentication 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.

   o  Access Providers SHALL NOT block users from accessing the external
      Internet using the SUBMISSION port.

   o  MUAs SHOULD use the SUBMISSION port for message submission.

   Note that the requirement for authentication, on the part of the MSA,
   thereby makes that MSA responsible for the ensuing traffic it

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   Figure 1 depicts examples of 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    smtp     /--------\      Destination
         +-------+    +-----+ port 25 |          |    +----------+
         | MUA.l | -> | MSA | ------> |          | -> |   MDA    |
         +-------+ 25 +-----+         | INTERNET | 25 +----------+
                   or    ^            |          |
                  587    \--------<---|---\      |
                                           ^ SUBMISSION
                                           | Port 587
                                       |  MUA.r |

                  Figure 1: Example of Port 587 Usage

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5. Message Submission Authentication Technologies

   There are many different technologies available to authenticate
   message submission transactions. These range from simple mechanisms
   like POP authorization before SMTP and IP Address access lists all
   the way to SMTP AUTH [4] and client side certificates using Transport
   Layer Security (TLS) [5]. Depending on the environment, each of these
   mechanisms can be more or less effective and convenient.
   Organizations are encouraged to choose the most secure approach that
   is practical.

   For example, SMTP AUTH [4] using a secure authentication method like
   CRAM-MD5 or DIGEST-MD5 may be sufficient. However, in some
   environments, it is impractical to use one of the secure methods,
   meaning that SMTP AUTH [4] would be transmitting the username and the
   password in clear text over insecure networks. This could allow
   attackers to listen for this traffic and steal account data. In these
   cases, using STARTTLS [5] to establish an encrypted channel for
   transmission of the SMTP AUTH [4] username and password would be
   preferred. Similarly, STARTTLS [5] with client side certificates
   could be used with the SMTP AUTH [4] EXTERNAL mechanism to achieve
   secure authentication.

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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 to permit such exchanges while reducing
   the likelihood that malicious mail will be transmitted.

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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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Normative References

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

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

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Informative References

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

   [4]  Myers, J., "SMTP Service Extension for Authentication", RFC
        2554, March 1999.

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

Authors' Addresses

   Carl Hutzler
   America Online
   12100 Sunrise Valley Drive
   Reston, VA

   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@brandenburg.com
   URI:   http://www.brandenburg.com/

   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/

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   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/

   Eric Allman
   Sendmail, Inc.
   6425 Christie Avenue, Suite 400
   Emeryville, CA  94608

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

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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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Full Copyright Statement

   Copyright (C) The Internet Society (2004). All Rights Reserved.

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   This document and the information contained herein is provided on an

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   Funding for the RFC Editor function is currently provided by the
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