mx.tucows.com.cust.hostedemail.com has no CNAME records.
Question and response
Question
dig mx.tucows.com.cust.hostedemail.com. NS
Answer
The response has no answer records.
Authority
cust.hostedemail.com. 186 SOA ns1.mdnsservice.com. hostmaster.mdnsservice.com. 2008042500 7200 7200 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0