Null MX
A single MX record with preference 0 and the target "." that declares a domain accepts no email (RFC 7505).
What it means
Publishing MX 0 . tells senders the domain takes no mail, so they fail at once instead of queuing and retrying for days. It must be the only MX record for the domain. RFC 7505 asks servers rejecting a recipient for this reason to reply 556 with enhanced code 5.1.10.
It also signals that the domain should not appear as a sender. Receivers may reject mail whose MAIL FROM or From domain has a null MX, using 550 and 5.7.27, since any bounce to it would be undeliverable.
For a parked or web-only domain, pair a null MX with v=spf1 -all and a DMARC p=reject record to close it to spoofing as well.
Example
example.com. 3600 IN MX 0 .Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes