Glossary
MX record
The DNS record that says which server accepts mail for a domain.
An MX record is the DNS entry that tells a sending server where to deliver mail for a domain. A domain can publish several, each with a priority number, and the lowest number is tried first. Without one there is nowhere for mail to go.
The MX lookup decides whether verification continues at all. If the domain does not exist, the answer is settled immediately — DNS_NXDOMAIN, and no mail server needs to be contacted. If the domain exists but publishes no MX record, mail may still be deliverable: the standard allows a sender to fall back to the domain’s A record, which is reported as IMPLICIT_MX rather than treated as a failure.
Two cases are worth knowing because they look alike and mean opposite things. A null MX — a single record pointing at . — is an explicit declaration that the domain accepts no mail at all, which is a definite answer and a fast one. An MX that exists but resolves to nothing (MX_UNRESOLVABLE) is a misconfiguration: somebody intended mail to work here and it does not.
DNS is also the cheapest place to catch a typo. A domain that does not resolve, or that resolves but sits one character away from a very common one, can be identified before any mail server is contacted — which is why LIKELY_DOMAIN_TYPO is its own signal rather than a footnote on a failure.
What the service returns
Reason codes from the closed vocabulary in the API response. These are the exact strings an integration writes an if against.
MX_FOUND | Mail exchangers found |
NULL_MX | Domain declares that it accepts no mail |
IMPLICIT_MX | Mail routed by implicit MX |
MX_UNRESOLVABLE | Mail exchanger does not resolve |
DNS_NXDOMAIN | Domain does not exist |
LIKELY_DOMAIN_TYPO | Domain looks like a typo |
See it on a real address.
The free checker returns the whole result — every stage, every reason code, nothing held back.