mx.duoservers.com has no CNAME records.
Question and response
Question
dig mx.duoservers.com. NS
Answer
The response has no answer records.
Authority
duoservers.com. 2525 SOA dns1.duoservers.com. hostmaster.duoservers.com. 1772473507 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0