A "Null MX" No Service Resource Record for Domains That Accept No Mail
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 efficienc
Normative requirements
Every sentence in this RFC carrying an RFC 2119 keyword, with the section it came from. 1 must, 3 should, 0 may.
3 MX Resource Records Specifying Null MX
- MUST NOTA domain that advertises a null MX MUST NOT advertise any other MX RR.
4.1 SMTP Server Benefits
- SHOULDWhen 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).
- SHOULDSMTP 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
- SHOULD NOTHence, mail systems SHOULD NOT publish a null MX record for domains that they use in RFC5321.MailFrom or RFC5322.From addresses.