MX record
The DNS record that names which servers accept mail for a domain, and in what order to try them.
A mail exchanger record maps a domain to the hostnames that handle its email, each with a priority number. Lower numbers are tried first. A domain with no MX record generally cannot receive mail at all, which makes this the cheapest hard signal in email validation.
MX lookups also reveal the actual mail provider behind a custom domain. A domain whose MX points at Google or Microsoft is running hosted mail; one pointing at a spam-filtering appliance is behind a gateway that may accept everything and filter later.
Because the record is public DNS, checking it costs nothing and touches no mail server. It is the step that separates a real business domain from one that was registered and parked.