Null MX
An MX record of the form "0 ." that explicitly declares a domain does not accept email.
A Null MX record is the single line `0 .` published as the MX for a domain. It is the standardised way to say "this domain sends no mail and accepts none, do not even try." Compliant sending MTAs will immediately bounce mail addressed to a Null MX domain with a permanent 556 error instead of sitting in a retry queue for days. Brands often use Null MX on apex domains they use only for web hosting, alongside a strict SPF (`v=spf1 -all`) and DMARC reject policy, to make spoofing visibly invalid.