Where you saw it: A verifier marking every address at the domain as undeliverable, a bounce reading no mail server, or an MX lookup returning nothing.
Why it happens
- The domain was registered for a website and never configured for mail, which is normal and correct for a domain nobody sends to.
- A DNS migration moved the zone and the MX records were not recreated.
- The mail provider was canceled and their records were removed with the account.
- The records were created on the wrong host, such as mail.example.com instead of the root.
- A typo in the MX target points at a hostname that does not resolve, which fails in the same way as having nothing.
The fix, in order
Confirm with a direct query
Run dig +short MX yourdomain.com. An empty result confirms it. Check your registrar's panel too, since a zone edited in the wrong place is a frequent cause.
Check whether an A record is catching the mail
Without MX records, senders fall back to the A record of the domain. If that points at a web server, mail is being delivered somewhere that is not listening, and it fails silently.
Publish the records your provider specifies
Take the hostnames and priorities from your mail provider's own documentation. Google Workspace, Microsoft 365 and every other platform publishes an exact list.
Get the priorities right
The lowest number is tried first. Several records at the same priority share load. Copy the provider's values rather than inventing your own.
Publish a null MX where a domain should never receive mail
A single record with priority 0 and a target of . tells senders the domain accepts no mail. That is a clear statement rather than an accident, and verifiers read it correctly.
dig +short MX example.comNo output means no mail exchanger, and that is the whole diagnosis.
How to know it worked
The short version
- dig +short MX yourdomain.com returns your provider's hostnames with their priorities.
- Each target resolves: run dig +short A on the hostname you published.
- Send a message from an outside address and confirm it arrives.
Questions people ask
Can a domain receive mail without MX records?
Sometimes, through the A record fallback, and you should not rely on it. Many senders and most verifiers treat a domain with no MX as undeliverable.
What is a null MX record?
A single MX record with priority 0 pointing at a dot. It declares that the domain accepts no mail at all, which stops senders trying and stops backscatter.
Does a missing MX record mean every address there is invalid?
In practice, yes. With nowhere to deliver, no address at that domain is reachable, which is why we mark the whole domain rather than probing each address.