# Implicit MX

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.

Date: 2026-09-10

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`](/docs/reasons/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](/glossary/catch-all-domain): 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.

Reference: https://spaw.co/glossary/implicit-mx