MX targets are hostnames with address records

RFC 2181 §10.3 is explicit: the target of an MX record must be a hostname with address records, not an alias. This check reports two violations of that — an MX whose target is a CNAME, and an MX whose target is an IP address written where a hostname belongs.

The CNAME case is graded as a failure because senders genuinely disagree about it. Some follow the alias and deliver; some reject the target as invalid and treat the domain as having no usable MX. That is not a fault you can test your way out of by sending yourself a message — it fails for a subset of the internet, and which subset depends on what software your correspondents happen to run.

An IP literal is simpler and more absolute. The field holds a domain name, so 192.0.2.25 in an MX record is a hostname made of digits and dots. It resolves to nothing, and nothing delivers.

Publish A and AAAA records at the MX hostname itself, so the name the MX points at is a real host rather than a signpost to one.

If your provider gave you a CNAME to point at, put the MX record on the target of that CNAME instead — the canonical name the alias resolves to. Setup instructions telling you to put an MX in front of a CNAME are common and wrong; the provider's own MX hostname is what belongs there.

For an IP literal, replace it with a hostname that resolves to that address. If the host has no name, give it one under your own domain and publish the address record — an MX host needs a name anyway, for its TLS certificate and its reverse DNS to be worth anything.