Null MX record

A DNS record (MX 0 .) that says a domain accepts no email at all (RFC 7505), so senders give up at once and list checks can drop its addresses.

A domain that publishes a single MX record with preference 0 and an empty host declares that it does not receive mail; senders should fail immediately instead of retrying. example.com publishes one. Without any MX record, mail falls back to the domain’s A record (RFC 5321), so a missing MX is not the same as a null MX. Domains used only for websites, redirects or parked names are good candidates: senders stop at once instead of retrying, and receivers may reject mail that claims to come from such a domain, because no reply could reach it.

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Related terms

MX record: A DNS record that names the mail servers accepting email for a domain, each with a preference number: lower numbers are tried first.
Hard bounce: A permanent delivery failure: the address or its domain does not exist or refuses mail.
SPF (Sender Policy Framework): A DNS record listing which servers may send email for your domain.

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