A "Null MX" No Service Resource Record for Domains That Accept No Mail
RFC 7505

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Internet Engineering Task Force (IETF)                         J. Levine
Request for Comments: 7505                          Taughannock Networks
Category: Standards Track                                      M. Delany
ISSN: 2070-1721                                               Apple Inc.
                                                               June 2015

 A "Null MX" No Service Resource Record for Domains That Accept No Mail

Abstract

   Internet mail determines the address of a receiving server through
   the DNS, first by looking for an MX record and then by looking for an
   A/AAAA record as a fallback.  Unfortunately, this means that the
   A/AAAA record is taken to be mail server address even when that
   address does not accept mail.  The No Service MX RR, informally
   called "null MX", formalizes the existing mechanism by which a domain
   announces that it accepts no mail, without having to provide a mail
   server; this permits significant operational efficiencies.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7505.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.

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RFC 7505                         Null MX                       June 2015

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  MX Resource Records Specifying Null MX  . . . . . . . . . . .   3
   4.  Effects of Null MX  . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  SMTP Server Benefits  . . . . . . . . . . . . . . . . . .   3
     4.2.  Sending Mail from Domains That Publish Null MX  . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   This document defines the No Service MX, informally called "null MX",
   as a simple mechanism by which a domain can indicate that it does not
   accept email.

   SMTP clients have a prescribed sequence for identifying a server that
   accepts email for a domain.  Section 5 of [RFC5321] covers this in
   detail; 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.
   Hence, this overloads a DNS record (that has a different primary
   mission) with an email service semantic.

   If a domain has no MX records, senders will attempt to deliver mail
   to the hosts at the addresses in the domain's A or AAAA records.  If
   there are no SMTP listeners at the A/AAAA addresses, message delivery
   will be attempted repeatedly for a long period, typically a week,
   before the sending Mail Transfer Agent (MTA) gives up.  This will
   delay notification to the sender in the case of misdirected mail and
   will consume resources at the sender.

   This document defines a null MX that will cause all mail delivery
   attempts to a domain to fail immediately, without requiring domains
   to create SMTP listeners dedicated to preventing delivery attempts.

2.  Conventions Used in This Document

   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].

Levine & Delany              Standards Track                    [Page 2]
RFC 7505                         Null MX                       June 2015