Email Submission Operations: Access and Accountability Requirements

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.

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

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.

   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] [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
   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
      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.

   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
   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.

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

   [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.
   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/
   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/

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