mx.tucows.com.cust.hostedemail.com has no NS records.
NS records for this zone are defined at cust.hostedemail.com.
NS records for this zone are defined at cust.hostedemail.com.
Question and response
Question
dig mx.tucows.com.cust.hostedemail.com. NS
Answer
The response has no answer records.
Authority
cust.hostedemail.com. 300 SOA ns1.mdnsservice.com. hostmaster.mdnsservice.com. 2008042500 7200 7200 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0