Implicit MX
The SMTP rule that when a domain has no MX records, senders deliver to the domain's own A or AAAA address as if it had an MX of preference 0.
What it means
RFC 5321 section 5.1 says that if an MX lookup returns no records, the domain is treated as having an implicit MX pointing to itself. Mail for user@example.com then goes to whatever address example.com resolves to, which may be a web server that does not run SMTP or should not receive mail.
The fallback applies only when there are no MX records at all; once any MX exists, address records are never used directly. SPF's mx mechanism does not apply the implicit rule either.
Domains that do not want mail should publish a null MX, which stops the fallback.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321: Simple Mail Transfer Protocol, section 5.1rfc-editor.org
- RFC 7208: Sender Policy Framework (SPF), section 5.4rfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes