mail.duoservers.com has no CNAME records.
Question and response
Question
dig mail.duoservers.com. NS
Answer
The response has no answer records.
Authority
duoservers.com. 1716 SOA dns1.duoservers.com. hostmaster.duoservers.com. 1772470506 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0