A "Null MX" No Service Resource Record for Domains That Accept No Mail
A single MX record that tells the world a domain accepts no email, so senders give up at once.
Levine, J. and M. Delany, "A "Null MX" No Service Resource Record for Domains That Accept No Mail", RFC 7505, DOI 10.17487/RFC7505, June 2015, <https://www.rfc-editor.org/info/rfc7505>.
@misc{rfc7505,
series = {Request for Comments},
number = 7505,
howpublished = {RFC 7505},
publisher = {RFC Editor},
doi = {10.17487/RFC7505},
url = {https://www.rfc-editor.org/info/rfc7505},
author = {J. Levine and M. Delany},
title = {{A "Null MX" No Service Resource Record for Domains That Accept No Mail}},
pagetotal = 6,
year = 2015,
month = jun,
} RFC text
Contents
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.
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].
The terms "RFC5321.MailFrom" and "RFC5322.From" are used as defined
in [RFC5598].
3. MX Resource Records Specifying Null MX
To indicate that a domain does not accept email, it advertises a
single MX RR (see Section 3.3.9 of [RFC1035]) with an RDATA section
consisting of preference number 0 and a zero-length label, written in
master files as ".", as the exchange domain, to denote that there
exists no mail exchanger for a domain. Since "." is not a valid host
name, a null MX record cannot be confused with an ordinary MX record.
The use of "." as a pseudo-hostname meaning no service available is
modeled on the SRV RR [RFC2782] where it has a similar meaning.
A domain that advertises a null MX MUST NOT advertise any other MX
RR.
4. Effects of Null MX
The null MX record has a variety of efficiency and usability
benefits.
4.1. SMTP Server Benefits
Mail often has an incorrect address due to user error, where the
address was mistranscribed or misunderstood, for example, to
alice@www.example.com, alice@example.org, or alice@examp1e.com rather
than alice@example.com. Null MX allows a mail system to report the
delivery failure when the user sends the message, rather than hours
or days later.
Senders of abusive mail often use forged undeliverable return
addresses. Null MX allows Delivery Status Notifications (DSNs) and
other attempted responses to such mail to be disposed of efficiently.
The ability to detect domains that do not accept email offers
resource savings to an SMTP client. It will discover on the first
sending attempt that an address is not deliverable, avoiding queuing
and retries.
When a submission or SMTP relay server rejects an envelope recipient
due to a domain's null MX record, it SHOULD use a 556 reply code
[RFC7504] (Requested action not taken: domain does not accept mail)
and a 5.1.10 enhanced status code (Permanent failure: Recipient
address has null MX).
A receiving SMTP server that chooses to reject email during the SMTP
conversation that presents an undeliverable RFC5321.MailFrom or
RFC5322.From domain can be more confident that for other messages a
subsequent attempt to send a DSN or other response will reach a
recipient SMTP server.
SMTP servers that reject mail because a RFC5321.MailFrom or
RFC5322.From domain has a null MX record SHOULD use a 550 reply code
(Requested action not taken: mailbox unavailable) and a 5.7.27
enhanced status code (Permanent failure: Sender address has null MX).
4.2. Sending Mail from Domains That Publish Null MX
Null MX is primarily intended for domains that do not send or receive
any mail, but have mail sent to them anyway due to mistakes or
malice. Many receiving systems reject mail that has an invalid
return address. Return addresses are needed to allow the sender to
handle message delivery errors. An invalid return address often
signals that the message is spam. Hence, mail systems SHOULD NOT
publish a null MX record for domains that they use in
RFC5321.MailFrom or RFC5322.From addresses. If a system nonetheless
does so, it risks having its mail rejected.
Operators of domains that do not send mail can publish Sender Policy
Framework (SPF) "-all" policies [RFC7208] to make an explicit
declaration that the domains send no mail.
Null MX is not intended to be a replacement for the null reverse-path
described in Section 4.5.5 of RFC 5321 and does not change the
meaning or use of a null reverse-path.
5. Security Considerations
Within the DNS, a null MX RR is an ordinary MX record and presents no
new security issues. If desired, it can be secured in the same
manner as any other DNS record using DNSSEC.
6. IANA Considerations
IANA has added the following entries to the "Enumerated Status Codes"
subregistry of the "Simple Mail Transfer Protocol (SMTP) Enhanced
Status Codes Registry".
Code: X.1.10
Sample Text: Recipient address has null MX
Associated basic status code: 556
Description: This status code is returned when the associated
address is marked as invalid using a null MX.
Reference: This document
Submitter: Authors of this document
Change controller: IESG
Code: X.7.27
Sample Text: Sender address has null MX
Associated basic status code: 550
Description: This status code is returned when the associated
sender address has a null MX, and the SMTP
receiver is configured to reject mail from such
sender (e.g., because it could not return a DSN).
Reference: This document
Submitter: Authors of this document
Change controller: IESG
7. References
7.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC7504] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504,
DOI 10.17487/RFC7504, June 2015,
<http://www.rfc-editor.org/info/rfc7504>.
7.2. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<http://www.rfc-editor.org/info/rfc5598>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<http://www.rfc-editor.org/info/rfc7208>.
Acknowledgements
We thank Dave Crocker for his diligent and lengthy shepherding of
this document, and members of the APPSAWG working group for their
constructive suggestions.
Authors' Addresses
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
United States
Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://jl.ly
Mark Delany
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
United States
Email: mx0dot@yahoo.com
AI ELI5
On this page
§1Why a Domain Needs to Say No Mail
An SMTP client finds a mail server for a domain in a set order, described in Section 5 of RFC 5321. It first looks up the domain's MX records. If there are none, it falls back to the domain's A or AAAA records. That overloads an address record, whose main job is something else, with a mail meaning.
So a domain with no MX records still gets delivery attempts to its web or other hosts. If nothing listens for SMTP there, the sending Mail Transfer Agent (MTA) tries again and again, typically for a week, before giving up. The sender is told late about misdirected mail and wastes resources in the meantime.
The RFC defines the No Service MX, informally called null MX, so that all delivery attempts fail immediately without the domain running an SMTP listener just to refuse mail.
If a house has no mailbox, the mail carrier might keep coming back for a week to check. A null MX is a clear note on the door that says do not bother.
§3The Null MX Record
A domain that accepts no email publishes a single MX record (defined in Section 3.3.9 of RFC 1035). Its data holds preference number 0 and a zero-length label as the exchange domain, written . in master files. That means there is no mail exchanger for the domain.
Because . is not a valid host name, a null MX cannot be mistaken for an ordinary MX record. The idea is modeled on the SRV record from RFC 2782, where a . target has a similar meaning of no service available.
A domain that advertises a null MX MUST NOT advertise any other MX record.
The record points to a mail server whose name is just a dot, which is not a real name at all. That is the way of saying: there is no mail server here.
§4What Happens When Mail Meets a Null MX
Mistyped addresses are common, such as alice@www.example.com, alice@example.org or alice@examp1e.com instead of alice@example.com. With a null MX, the mail system can report the failure when the user sends the message, rather than hours or days later. An SMTP client learns on its first attempt that the address is undeliverable, so it avoids queuing and retries.
Abusers often forge return addresses that cannot receive mail. A null MX lets Delivery Status Notifications (DSNs) and other replies to such mail be disposed of efficiently. A receiving server that rejects mail from an undeliverable sender domain can also be more confident that a later DSN to a real sender will reach a working server.
When a submission or relay server rejects a recipient because of a null MX, it SHOULD use reply code 556 (from RFC 7504) and enhanced status code 5.1.10. When a server rejects mail because the sender's domain has a null MX, it SHOULD use reply code 550 and enhanced status code 5.7.27.
If you address a letter to a house with the no-mail sign, the post office hands it straight back to you today. You do not wait a week to find out.
§4.2Domains That Send Mail
Null MX is meant mainly for domains that neither send nor receive mail, but still get mail because of mistakes or malice. Many receivers reject mail with an invalid return address, and an invalid return address often signals spam. So mail systems SHOULD NOT publish a null MX for domains they use in RFC5321.MailFrom or RFC5322.From addresses. If they do, their mail risks being rejected.
Operators of domains that send no mail can also publish Sender Policy Framework (SPF) -all policies (RFC 7208) to declare that. Null MX does not replace the null reverse-path of Section 4.5.5 of RFC 5321 and does not change how that is used.
Do not put the no-mail sign on a house you send letters from. People who get a letter with a no-return-address house on it may toss it out.
§5Security Considerations
Inside the DNS, a null MX is an ordinary MX record, so it adds no new security issues. If wanted, it can be secured like any other DNS record using DNSSEC.
The sign is written in the same ink as every other DNS record, so it can be protected the same way.
§6New Enhanced Status Codes
IANA added two entries to the Enumerated Status Codes subregistry of the SMTP Enhanced Status Codes registry. Code X.1.10 (sample text: Recipient address has null MX) goes with basic status code 556 and is returned when an address is marked invalid by a null MX.
Code X.7.27 (sample text: Sender address has null MX) goes with basic status code 550. It is returned when the sender address has a null MX and the SMTP receiver is configured to reject mail from such senders, for example because it could not return a DSN.
Two new error labels were added to the official list, one for mail sent to a no-mail domain and one for mail claiming to come from one.
- 556 / 5.1.10Recipient address has null MX — Used when a recipient's domain publishes a null MX.
- 550 / 5.7.27Sender address has null MX — Used when mail is rejected because the sender's domain publishes a null MX.