Why an MX record must not point to a CNAME
An MX record tells the world which servers accept mail for your domain. Its target must be a hostname that has its own address records (A or AAAA). If the target is instead a CNAME, an alias for another name, you have broken the rules in RFC 2181 and RFC 1912.
Why it matters
Some mail servers follow the alias and deliver anyway, so the problem can go unnoticed for months. Others treat it as a configuration error and refuse to deliver, or defer the message until it bounces. The result is mail that arrives from some senders and silently fails from others, which is hard to diagnose.
Common causes
- A mail provider told you to point
mail.example.comat their hostname with a CNAME, and the MX then points atmail.example.com. - A server was renamed and its old name was turned into an alias.
- The MX target was copied from a record that happens to be an alias.
How to fix it
- Run DNSLint on your domain and note which MX targets it flags.
- Look up the flagged hostname and follow the CNAME to the name it finally resolves to.
- Change the MX record to point directly at that final hostname, or replace the CNAME with A and AAAA records that hold the same addresses.
- Wait for the old record’s TTL to expire, then run the check again.
If your mail provider gave you the MX hostnames to use, use them exactly as given. Providers publish those names with address records on purpose.