mail.duoservers.com has no MX records.
Question and response
Question
dig mail.duoservers.com. MX
Answer
The response has no answer records.
Authority
duoservers.com. 2560 SOA dns1.duoservers.com. hostmaster.duoservers.com. 1772470504 16384 2048 1048576 2560
Additional
The response has no additional records.