What MX record means
An MX record maps a domain to a mail exchanger and a preference: example.com. 3600 IN MX 10 mx1.example.net. Lower preference is tried first, hosts sharing a preference are tried in any order, and that is how load is spread across two equal servers. RFC 5321 §5.1 governs how a sender resolves and uses the set.
The target must be a hostname with an A or AAAA record. Not an IP address — the record type has no syntax for one — and not a CNAME, which RFC 2181 forbids as an MX target. A domain publishing no MX at all falls back to its own address record, the implicit MX, which is how a website ends up receiving mail nobody configured it for; a domain that receives no mail should say so with a null MX, the record 0 . from RFC 7505.
Two failures look identical from outside: a target that no longer resolves, and a target that resolves to an address refusing connections on port 25. Both defer mail and both bounce days later. Resolving every target and opening a connection to each is the check worth running — every target, because the secondary nobody has tested since the day it was added is where mail will queue when the primary is down.
See it on your own domain. Every term in this glossary is something one of the eleven checks reads and reports.
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Related checks, definitions and guides.
- A record What an MX target has to resolve to.
- Bounce What an unreachable mail exchanger eventually produces.
- MX lookup Resolve a domain's mail exchangers and their preferences.
- SMTP server test Open a connection to each host the MX set names.
- How MX records route mail Preferences, fallbacks and the null MX.