"Internet mail determines the address of a receiving server through the DNS@ first by looking for an MX record and then by looking for an A/AAAA record as a fallback. Unfortunately@ this means that the A/AAAA record is taken to be mail server address even when that address does not accept mail. The No Service MX RR@ informally called ""null MX""@ formalizes the existing mechanism by which a domain announces that it accepts no mail@ without having to provide a mail server; this permits significant operational efficiencies."
RFC 7505-2015 history
2015RFC 7505-2015 "A ""Null MX"" No Service Resource Record for Domains That Accept No Mail"