An MX record (mail exchanger record) is a DNS record that names the server that accepts email for a domain, with a priority number. Lower numbers are tried first.
gmail.com. MX 5 gmail-smtp-in.l.google.com.
What it tells you about an address
- No MX record (and no fallback A record): mail to the domain can never be delivered.
- Null MX (a single MX pointing to
.): the domain explicitly refuses all email. See null MX. - MX shared with throwaway domains: a strong sign the domain is a disposable email service.
Look up any domain with the free MX lookup.