What causes “invalid MX record”?

"Invalid MX record" indicates mail routing misconfiguration:

Common causes:

MX points to non-existent host: The hostname in MX has no A record. Syntax errors: Malformed MX record. Missing trailing dot: FQDN formatting issue. Circular references: MX points to CNAME.

Technical issues:

MX record with priority but no hostname. Hostname that does not resolve. TTL or propagation problems.

Diagnosis:

Query MX records: dig MX domain.com. Verify each MX hostname resolves. Check for CNAME conflicts.

Who can fix it:

Only the domain owner or administrator. Not something the sender can resolve. May need to contact recipient via other means.

Invalid MX is a broken signpost. The directions exist but lead nowhere.