What is an MX record?

An MX (Mail Exchange) record tells the world which server or servers are responsible for receiving e-mail on behalf of a domain, and in what order to try them. Without one, mail addressed to that domain has nowhere defined to arrive.

No MX isn't automatically wrong

A domain that only serves a website, or is only ever used to send mail rather than receive it, can legitimately have no MX record at all. RFC 5321 has a receiving server fall back to the domain's own A/AAAA record when no MX exists, but relying on that fallback is rare and rarely deliberate in a modern setup.

A null MX is a deliberate statement

RFC 7505 defines a "null MX", a single record with preference 0 and target ".", as an explicit way to say "this domain accepts no mail at all". That's different from simply having no MX record: it tells a sending server not to bother retrying. It's only valid as the sole MX record; mixed in alongside real ones, it contradicts them.

What this checks beyond presence

Each target is checked for things that are easy to get wrong and hard to notice: whether it's a bare IP address or a CNAME (both invalid per RFC 1035 and RFC 2181 respectively, even though many mail servers tolerate a CNAME anyway), whether it resolves at all, over which address families, and whether it has reverse DNS. That last one enforces nothing by itself, but a fair number of receiving servers weigh it when deciding whether to trust an incoming connection.

One live check, on the primary host only

Every other check on this page is pure DNS. This one actually connects: to the lowest-preference MX host, on port 25, to confirm it speaks SMTP and negotiates STARTTLS - the same live-check approach the DANE check already applies to this exact host. Only the primary host gets this, to keep the check to one live connection rather than one per MX record. Port 587 (mail submission) is checked too, but purely informationally: it's not something an MX record requires, and plenty of legitimate setups don't offer it here at all.