MX record
An MX record is the DNS entry naming the server that handles mail for a domain. If a domain has no MX record, nothing sent to any address at that domain can be delivered.
It is the cheapest meaningful check in verification, and it is decisive in one direction only. No MX record means no mailbox, and the whole domain can be dropped from a list in one query. An MX record present means mail is accepted somewhere at the domain — it says nothing about whether the specific mailbox exists.
The record's value also tells you who runs the mail. An MX pointing at Google or Microsoft means a large provider with well-documented behaviour; an obscure host often means a small business on shared hosting, which tends to be both stricter about volume and more likely to accept everything.
- No MX
- Nothing at this domain can receive mail. Drop every row for the domain.
- MX present
- The domain accepts mail. The mailbox still has to be checked separately.
Goes deeper: Email Verification, Explained Properly