Definition
A null MX is an MX record with priority 0 and a single dot as the hostname:
example.com. 3600 IN MX 0 .
It is defined in RFC 7505. The record says, in effect, "this domain does not receive mail." It must be the only MX record on the domain.
Without it, a domain with no MX records is ambiguous. RFC 5321 says senders may fall back to the domain's A or AAAA record and try to deliver there. That leads to mail piling up against web servers that were never meant to receive it, and to slow failures for the sender. A null MX makes the answer explicit and immediate.
Domain owners use null MX for web-only domains, parked domains, short links and domains used purely for redirects.
Why it matters
Any address at a null MX domain cannot receive email. Sending to one produces a hard bounce, typically with an enhanced status code in the 5.1.10 range or a similar "domain does not accept mail" message, depending on the sending software.
For list quality, null MX is a gift: a definite answer without any guesswork. It often shows up when a company has moved to a new domain and kept the old one only for its website, or when someone typed their company's marketing domain instead of their email domain.
How SimpleCold handles it
Domain and MX checks run before any mail server connection in SimpleCold's pipeline. An address whose domain cannot receive mail comes back Undeliverable with the status no_mail_server and a failType of permanent, so you can suppress it straight away.
Look up a domain's MX records, including null MX, with the free MX lookup.