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Implicit MX

published · September 10, 2026

Implicit MX is RFC 5321's fallback: a domain with no MX record but with an A or AAAA record is treated as its own mail exchanger.

Implicit MX is RFC 5321's fallback: a domain with no MX record but with an A or AAAA record is treated as its own mail exchanger, and mail is delivered to that host. The rule dates from a time when a domain's web server and mail server were the same machine.

What Spaw answers

mx_implicit is true and the reason is implicit_mx. The verdict is risky, not undeliverable, and the risk score carries +30 under "web-only domain, no MX record".

The judgement behind that weight is plain: by the letter of the standard the address is deliverable, and in practice the host at a modern domain's A record is a web server that will refuse the connection. Spaw will not call it deliverable and will not call it dead, because both would be claims the DNS does not support.

What to do with it

Treat it the way you treat a catch-all: a signup can proceed, a bulk send probably should not. If the address matters, ask the person to confirm it another way — an implicit-MX domain that really does receive mail is rare enough to be worth checking rather than assuming.

Related

markdown version: /glossary/implicit-mx.md

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