mx.socket.net.cust.b.hostedemail.com has no CNAME records.
Question and response
Question
dig mx.socket.net.cust.b.hostedemail.com. NS
Answer
The response has no answer records.
Authority
cust.b.hostedemail.com. 300 SOA ns1.mdnsservice.com. hostmaster.mdnsservice.com. 953732100 10001 7200 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0