Null MX record
A null MX record is a specific MX record -- a single record pointing to the root domain (".") -- that a domain publishes to explicitly declare it doesn't accept email at all, standardized in RFC 7505. Before null MX existed, a domain that sent no mail had no standard way to say so: it would either publish no MX record, which falls back to trying the domain's own A/AAAA record for SMTP delivery under RFC 5321's implicit MX rule, or leave mail handling undefined, leaving senders unable to tell a missing server from a misconfiguration. Publishing a null MX tells sending servers to reject outgoing mail to that domain immediately, useful for domains used only for hosting or infrastructure, where no one should ever email that name. AdminKit's MX lookup tool reports a null MX explicitly, rather than showing it as a missing or broken record.
By AdminKit team, Netbase JSC · Updated