What is a null MX record?
Also called: null MX record, RFC 7505
A null MX is a single MX record with priority 0 and a target of ".", published to declare that a domain accepts no email at all. Defined in RFC 7505, it lets sending servers fail immediately instead of retrying, and it marks every address at that domain as undeliverable.
Without it, a domain that never receives mail is ambiguous: senders may fall back to its A record and try to deliver there, then retry for days. A null MX ends that with an immediate permanent failure.
Domains that only send, such as a subdomain used for notifications, and parked domains are the typical users. It is worth publishing on any domain you own that should never receive mail, because it also discourages spammers from forging addresses there.
For list cleaning it is the clearest signal there is. Every address at a null-MX domain will bounce.
Example
example.com. IN MX 0 .How SimpleVerifier handles it
A null MX gives no mail host to connect to, so addresses at that domain are returned as invalid · No mail server.
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Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms