mail.codero.net has no CNAME records.
Question and response
Question
dig mail.codero.net. NS
Answer
The response has no answer records.
Authority
codero.net. 6388 SOA ns1.codero.com. hostmaster.codero.com. 97 7200 1800 1209600 46200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0