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MX_IMPLICIT_A
info
This domain has no MX records, so mail is delivered to whatever runs at its address — usually the web server, and usually by accident.
An observation. Whether it matters depends on what the domain is for.
How we know
mx.test
The same records are correct or broken depending on what the domain is for: nomail is a deliberate null MX, and the only difference between the passing and failing cases is what the caller says the domain does. Also carries the exchanges that cannot receive anything — an address written where a name belongs, an alias RFC 2181 forbids, a name with no address — plus an IPv6-only exchange, which is legitimate and would be failed by a check that reads a missing A record as a missing address.