Question: mx is a cname (illegal)?
Answer: 2 MX to CNAME Is Illegal. First, sendmail looks up hostA and gets an MX record pointing to mailhub. Because there is only a single MX record, sendmail considers mailhub to be official. Next, mailhub is looked up to find an A or AAAA record (IP address), but instead a CNAME (nfsmast) is returned.