What is an MX record?

Each record carries a priority, and sending servers try the lowest number first. Several records give a domain redundancy when one server is unavailable.

For verification it is the cheap first filter. No MX and no A record fallback means nothing can be delivered, and the check stops there without opening a connection.

Reading the MX host also tells you who actually runs the mail. A domain pointing at Google or Outlook behaves like that provider during verification, which is how a verifier predicts whether it is about to be throttled.

Run dig +short MX followed by a domain and you'll get one line per mail server. A company on Google Workspace typically returns 1 smtp.google.com, or on older setups five lines running from aspmx.l.google.com to several alt hosts. Microsoft 365 tenants return a single host ending in mail.protection.outlook.com.

The number in front is the preference. Senders try the lowest value first and move up only when that host can't be reached. Two records sharing the same number split the load between them, which is how larger sites spread incoming mail across machines.

A third answer worth recognizing is a filtering service. When the MX points at a security gateway instead of Google or Microsoft, a separate company's servers sit in front of the real mailboxes. That gateway may accept every recipient and sort them out later, so a 250 from it tells you less than one from the mail host itself.

A domain can publish a record that says it takes no mail at all. It's written as preference 0 with a single dot for the host, and it's defined in RFC 7505. Parked domains and send-only domains use it. If you see one, the address is undeliverable and no connection is needed to know it.

Some mistakes look fine in a DNS panel and still fail. An MX has to name a host, never an IP address. It also shouldn't point at a CNAME alias, because the standards forbid it and some sending servers refuse to follow one. And when the named host has no address record of its own, you get an MX that exists on paper and leads nowhere.

If you manage a domain, check yours after any hosting change. Old MX lines left behind from a previous provider keep receiving a share of your mail, and that server will bounce it.