MX targets are hostnames with address records
mail.mx-targets
RFC 2181 §10.3 forbids an MX target being a CNAME. Some senders follow it anyway, others reject the domain outright — so it fails for a subset of your mail.
A longer write-up of this check has not been published yet. The report itself shows the raw evidence behind every result, which is usually the fastest way to see what was measured.
Run this check on a domain
MX targets are hostnames with address records is one of 49 checks in every report, alongside delegation, mail authentication, TLS and registration.