The format of a null MX record
Per RFC 7505, a null MX record looks like 0 . — priority 0, target a single dot representing the DNS root rather than an actual hostname. A compliant sending server that encounters this should immediately treat the domain as permanently unable to receive mail, without attempting any SMTP connection at all.
Why this matters for verification pipelines
A verification tool that correctly recognizes a null MX record can immediately classify an address at that domain as undeliverable without wasting an SMTP connection attempt — treating it the same, confidently, as a domain with no MX record, but with explicit confirmation rather than inference from absence.
Null MX vs. a domain that simply has no MX record
Technically different, practically similar: a domain with genuinely no MX record and no null MX declaration is still undeliverable in practice (per RFC 5321, some senders fall back to the A record, but this isn’t guaranteed or standard), while a null MX is an unambiguous, intentional statement. Most verification tools treat both outcomes the same way operationally — “no mail service here” — even though only one of them is an explicit RFC 7505 declaration.
Where you’ll actually encounter null MX records
Primarily on domains or subdomains used exclusively for non-email purposes — a company’s main marketing domain might have a null MX on a specific subdomain used only for a web application, while the root domain still accepts mail normally through its own separate MX records.