Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Intended status: Standards Track                               M. Delany
Expires: July 7, 2014                                         Apple Inc.
                                                         January 3, 2014

       A NULL MX Resource Record for Domains that Accept No Mail


   When the 5321.MailFrom domain in an e-mail message has a DNS MX
   Resource Record (RR), it is making an explicit statement that it is
   willing to accept email.  However, when the domain has just a DNS A
   or AAAA RR, there mail clients cannot easily tell whether the domain
   accepts mail, as many hosts on the Internet advertise an A or AAAA RR
   regardless of whether they want to accept email.

   The NULL MX RR formalizes the existing mechanism by which a domain
   announces that it accepts no mail.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 7, 2014.

Copyright Notice

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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SMTP server benefits  . . . . . . . . . . . . . . . . . . . .   3
   3.  Parallel Considerations . . . . . . . . . . . . . . . . . . .   3
   4.  The NULL MX Resource Record . . . . . . . . . . . . . . . . .   4
   5.  Domains that do not send mail . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Inforrmative References . . . . . . . . . . . . . . . . .   5
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   5
     A.1.  Change from -01 to -02  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   This document formally defines the "NULL MX" as a simple mechanism by
   which a domain can indicate that it will never accept email.

   SMTP clients have a prescribed sequence for resolving how to deliver
   email to a domain.  Section 5 of [RFC5321] covers this in detail, but
   in essence the SMTP client first looks up a DNS MX RR and if that is
   not found it falls back to looking up a DNS A or AAAA RR.

   Many domains do not accept email, but do have A or AAAA records.  If
   they have no MX records, senders will attempt to deliver mail to
   those A or AAAA records.

   If there is no SMTP listener at that address, the message will be
   attempted repeatedly for a long period, typically a week, before the
   sending MTA gives up.  This will delay notification to the sender in
   the case of misdirected mail, and will consume resources at the

   If the domain has an SMTP listener at that address that rejects all
   connections (for instance with a 554 response as a connection-opening
   response) or has MX records pointing to such a listener then the
   sender will be notified in a timely fashion, but resources
   (generating a bounce) will still be consumed by the sender and it
   requires additional services to be provided which provide little
   benefit to the domain.

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   These resource usage problems are exacerbated when large volumes of
   email are sent using forged email addresses in a domain which does
   not accept email as its envelope sender, causing large numbers of
   bounces to be generated and to consume large amounts of resources at
   the sender of the bounces.

   This document defines a NULL MX that will cause all mail delivery
   attempts to a domain to fail immediately, without any reconfiguration
   of existing MTAs.

2.  SMTP server benefits

   Being able to detect domains that never accept email offers many
   resource savings to an SMTP server.  In the first instance, it can
   choose to reject email during the SMTP conversation that does not
   present a deliverable 5321.MailFrom domain.

   In the second instance, if an SMTP server accepts an email, it can be
   confident that an attempt to send a non-delivery email will likely be
   answered by another SMTP server.  This greatly helps to reduce non-
   delivery queues.  This contrasts greatly with the current situation
   where a non-delivery email for, e.g.,, will sit in
   the queue for a full queue lifetime as SMTP connection attempts to simply time out.

3.  Parallel Considerations

   Clearly the perpetrators of abusive mail can adapt such that the
   "vast class of email" that this mechanism helps identify, simply move
   over to using 5321.MailFrom domains that have valid MX RRs.

   While this is true, the direct benefits to the SMTP server still
   apply.  When an SMTP server queues a non-delivery email, the target
   domain will accept the email or give a definitive rejection so the
   queue entry will be removed promptly, thus keeping the queues short.

   There is also a fair amount of mail that is just misaddressed by
   people who mistranscribed or misunderstood an e-mail address, for
   example, or rather than  NULL MX allows a mail system to report the
   delivery failure when the user sends the message, rather than hours
   or days later.

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4.  The NULL MX Resource Record

   To indicate that a domain never accepts email, it advertises a single
   MX RR with a RDATA section consisting of preference number 0, and a
   dot, i.e., the DNS root, as the mail exchanger domain, to denote that
   there exists no mail exchanger for a domain.  (The DNS root is not a
   valid host name, which avoids any possibility that a NULL MX record
   could be confused with an ordinary MX record.)

   The interpretation of a NULL MX RR only applies when the domain has a
   single MX RR.  If a domain advertises multiple MX RRs including a
   NULL MX, the interpretation is as described in RFC5321.

5.  Domains that do not send mail

   An SMTP server when presented with an "I never accept email" MX might
   decline to accept such email as it knows that a response or non-
   delivery notice will never be accepted, and that legitimate mail
   rarely comes from domains that do not accept replies.

   SMTP servers that reject mail because a MAIL FROM domain has a NULL
   MX record should use a 550 reply code.

   Although NULL MX may imply that a domain sends no mail, it does not
   sat so explicitly.  Operators may want to publish SPF [RFC4408] -ALL
   policies to make an explicit statement.

6.  Security Considerations

   SMTP mail is inherently insecure in that it is feasible for even
   fairly casual users to negotiate directly with SMTP servers.  This
   proposal is about eliminating one small section of SMTP insecurity.

   In the unlikely event that a domain legitimately sends email but
   never wants to receive email, SMTP servers that reject mail from
   domains that advertise a NULL MX risk losing email from those
   domains.  Note that the normal way to send mail for which a sender
   wants no responses remains unchanged, by using an empty 5321.MailFrom

   Within the DNS, a NULL MX RR is an ordinary MX record and presents no
   new security issues.

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7.  References

7.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

7.2.  Inforrmative References

   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1", RFC
              4408, April 2006.

Appendix A.  Change Log

   *NOTE TO RFC EDITOR: This section may be removed upon publication of
   this document as an RFC.*

A.1.  Change from -01 to -02

   Note that SPF -ALL is how you say you send no mail.

Authors' Addresses

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 831 480 2300

   Mark Delany
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014


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