Free email-authentication toolkit
See which servers receive mail for a domain, in priority order — and whether the setup is valid DNS. We check the three things that quietly break delivery: a null MX, an MX pointing at an alias, and a host that no longer resolves. No signup, no backend: everything runs right here in your browser.
Enter a domain to list its mail exchangers by priority and check each host.
The basics, briefly
Mail is not automatically undeliverable. RFC 5321 tells a sender with no MX to fall back to the domain's A or AAAA address and try to deliver there. That is a deliberate setup for some small domains and an oversight for others, and DNS alone cannot tell you which — so this tool reports it as a fact, not as an error.
A single record reading 0 . is the RFC 7505 way of saying this domain accepts no mail at all. It is the correct configuration for a domain you only send from, or never use for email — and it is useful, because senders reject immediately instead of retrying for days. If you publish one, pair it with a DMARC record at p=reject; the DMARC checker shows what you have today.
RFC 2181 forbids it: an MX target must be a hostname with an address record, not an alias. The trouble is that many senders follow the alias anyway, so the misconfiguration works for most mail and fails for one sender — which is exactly the kind of fault that survives for years. This tool resolves every MX host and tells you when one is an alias.