mail.cdmon.com has no CNAME records.
Question and response
Question
dig mail.cdmon.com. NS
Answer
The response has no answer records.
Authority
cdmon.com. 566 SOA ns-1740.cdmon.com. cdmon-hostmaster.cdmon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0