7Internet Engineering Task Force (IETF) J. Levine
8Request for Comments: 7505 Taughannock Networks
9Category: Standards Track M. Delany
10ISSN: 2070-1721 Apple Inc.
14 A "Null MX" No Service Resource Record for Domains That Accept No Mail
18 Internet mail determines the address of a receiving server through
19 the DNS, first by looking for an MX record and then by looking for an
20 A/AAAA record as a fallback. Unfortunately, this means that the
21 A/AAAA record is taken to be mail server address even when that
22 address does not accept mail. The No Service MX RR, informally
23 called "null MX", formalizes the existing mechanism by which a domain
24 announces that it accepts no mail, without having to provide a mail
25 server; this permits significant operational efficiencies.
29 This is an Internet Standards Track document.
31 This document is a product of the Internet Engineering Task Force
32 (IETF). It represents the consensus of the IETF community. It has
33 received public review and has been approved for publication by the
34 Internet Engineering Steering Group (IESG). Further information on
35 Internet Standards is available in Section 2 of RFC 5741.
37 Information about the current status of this document, any errata,
38 and how to provide feedback on it may be obtained at
39 http://www.rfc-editor.org/info/rfc7505.
43 Copyright (c) 2015 IETF Trust and the persons identified as the
44 document authors. All rights reserved.
46 This document is subject to BCP 78 and the IETF Trust's Legal
47 Provisions Relating to IETF Documents
48 (http://trustee.ietf.org/license-info) in effect on the date of
49 publication of this document. Please review these documents
50 carefully, as they describe your rights and restrictions with respect
51 to this document. Code Components extracted from this document must
52 include Simplified BSD License text as described in Section 4.e of
53 the Trust Legal Provisions and are provided without warranty as
54 described in the Simplified BSD License.
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60RFC 7505 Null MX June 2015
65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
66 2. Conventions Used in This Document . . . . . . . . . . . . . . 2
67 3. MX Resource Records Specifying Null MX . . . . . . . . . . . 3
68 4. Effects of Null MX . . . . . . . . . . . . . . . . . . . . . 3
69 4.1. SMTP Server Benefits . . . . . . . . . . . . . . . . . . 3
70 4.2. Sending Mail from Domains That Publish Null MX . . . . . 4
71 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
72 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
73 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
74 7.1. Normative References . . . . . . . . . . . . . . . . . . 5
75 7.2. Informative References . . . . . . . . . . . . . . . . . 6
76 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
81 This document defines the No Service MX, informally called "null MX",
82 as a simple mechanism by which a domain can indicate that it does not
85 SMTP clients have a prescribed sequence for identifying a server that
86 accepts email for a domain. Section 5 of [RFC5321] covers this in
87 detail; in essence, the SMTP client first looks up a DNS MX RR, and,
88 if that is not found, it falls back to looking up a DNS A or AAAA RR.
89 Hence, this overloads a DNS record (that has a different primary
90 mission) with an email service semantic.
92 If a domain has no MX records, senders will attempt to deliver mail
93 to the hosts at the addresses in the domain's A or AAAA records. If
94 there are no SMTP listeners at the A/AAAA addresses, message delivery
95 will be attempted repeatedly for a long period, typically a week,
96 before the sending Mail Transfer Agent (MTA) gives up. This will
97 delay notification to the sender in the case of misdirected mail and
98 will consume resources at the sender.
100 This document defines a null MX that will cause all mail delivery
101 attempts to a domain to fail immediately, without requiring domains
102 to create SMTP listeners dedicated to preventing delivery attempts.
1042. Conventions Used in This Document
106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
108 document are to be interpreted as described in [RFC2119].
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119 The terms "RFC5321.MailFrom" and "RFC5322.From" are used as defined
124 To indicate that a domain does not accept email, it advertises a
125 single MX RR (see Section 3.3.9 of [RFC1035]) with an RDATA section
126 consisting of preference number 0 and a zero-length label, written in
127 master files as ".", as the exchange domain, to denote that there
128 exists no mail exchanger for a domain. Since "." is not a valid host
129 name, a null MX record cannot be confused with an ordinary MX record.
130 The use of "." as a pseudo-hostname meaning no service available is
131 modeled on the SRV RR [RFC2782] where it has a similar meaning.
133 A domain that advertises a null MX MUST NOT advertise any other MX
138 The null MX record has a variety of efficiency and usability
1414.1. SMTP Server Benefits
143 Mail often has an incorrect address due to user error, where the
144 address was mistranscribed or misunderstood, for example, to
145 alice@www.example.com, alice@example.org, or alice@examp1e.com rather
146 than alice@example.com. Null MX allows a mail system to report the
147 delivery failure when the user sends the message, rather than hours
150 Senders of abusive mail often use forged undeliverable return
151 addresses. Null MX allows Delivery Status Notifications (DSNs) and
152 other attempted responses to such mail to be disposed of efficiently.
154 The ability to detect domains that do not accept email offers
155 resource savings to an SMTP client. It will discover on the first
156 sending attempt that an address is not deliverable, avoiding queuing
159 When a submission or SMTP relay server rejects an envelope recipient
160 due to a domain's null MX record, it SHOULD use a 556 reply code
161 [RFC7504] (Requested action not taken: domain does not accept mail)
162 and a 5.1.10 enhanced status code (Permanent failure: Recipient
163 address has null MX).
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175 A receiving SMTP server that chooses to reject email during the SMTP
176 conversation that presents an undeliverable RFC5321.MailFrom or
177 RFC5322.From domain can be more confident that for other messages a
178 subsequent attempt to send a DSN or other response will reach a
179 recipient SMTP server.
182 RFC5322.From domain has a null MX record SHOULD use a 550 reply code
183 (Requested action not taken: mailbox unavailable) and a 5.7.27
184 enhanced status code (Permanent failure: Sender address has null MX).
1864.2. Sending Mail from Domains That Publish Null MX
188 Null MX is primarily intended for domains that do not send or receive
189 any mail, but have mail sent to them anyway due to mistakes or
190 malice. Many receiving systems reject mail that has an invalid
191 return address. Return addresses are needed to allow the sender to
192 handle message delivery errors. An invalid return address often
193 signals that the message is spam. Hence, mail systems SHOULD NOT
194 publish a null MX record for domains that they use in
195 RFC5321.MailFrom or RFC5322.From addresses. If a system nonetheless
196 does so, it risks having its mail rejected.
198 Operators of domains that do not send mail can publish Sender Policy
199 Framework (SPF) "-all" policies [RFC7208] to make an explicit
200 declaration that the domains send no mail.
202 Null MX is not intended to be a replacement for the null reverse-path
203 described in Section 4.5.5 of RFC 5321 and does not change the
204 meaning or use of a null reverse-path.
2065. Security Considerations
208 Within the DNS, a null MX RR is an ordinary MX record and presents no
209 new security issues. If desired, it can be secured in the same
210 manner as any other DNS record using DNSSEC.
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2316. IANA Considerations
233 IANA has added the following entries to the "Enumerated Status Codes"
234 subregistry of the "Simple Mail Transfer Protocol (SMTP) Enhanced
235 Status Codes Registry".
238 Sample Text: Recipient address has null MX
239 Associated basic status code: 556
240 Description: This status code is returned when the associated
241 address is marked as invalid using a null MX.
242 Reference: This document
243 Submitter: Authors of this document
244 Change controller: IESG
247 Sample Text: Sender address has null MX
248 Associated basic status code: 550
249 Description: This status code is returned when the associated
250 sender address has a null MX, and the SMTP
251 receiver is configured to reject mail from such
252 sender (e.g., because it could not return a DSN).
253 Reference: This document
254 Submitter: Authors of this document
255 Change controller: IESG
2597.1. Normative References
261 [RFC1035] Mockapetris, P., "Domain names - implementation and
262 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
263 November 1987, <http://www.rfc-editor.org/info/rfc1035>.
265 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
266 Requirement Levels", BCP 14, RFC 2119,
267 DOI 10.17487/RFC2119, March 1997,
268 <http://www.rfc-editor.org/info/rfc2119>.
270 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
271 DOI 10.17487/RFC5321, October 2008,
272 <http://www.rfc-editor.org/info/rfc5321>.
274 [RFC7504] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504,
275 DOI 10.17487/RFC7504, June 2015,
276 <http://www.rfc-editor.org/info/rfc7504>.
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284RFC 7505 Null MX June 2015
2877.2. Informative References
289 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
290 specifying the location of services (DNS SRV)", RFC 2782,
291 DOI 10.17487/RFC2782, February 2000,
292 <http://www.rfc-editor.org/info/rfc2782>.
294 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
295 DOI 10.17487/RFC5598, July 2009,
296 <http://www.rfc-editor.org/info/rfc5598>.
298 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
299 Authorizing Use of Domains in Email, Version 1", RFC 7208,
300 DOI 10.17487/RFC7208, April 2014,
301 <http://www.rfc-editor.org/info/rfc7208>.
305 We thank Dave Crocker for his diligent and lengthy shepherding of
306 this document, and members of the APPSAWG working group for their
307 constructive suggestions.
314 Trumansburg, NY 14886
317 Phone: +1 831 480 2300
318 Email: standards@taugh.com
328 Email: mx0dot@yahoo.com
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