Apex domains cannot have CNAME records.
Question and response
Question
dig cust.b.hostedemail.com. NS
Answer
cust.b.hostedemail.com. 5 NS ns3.mdnsservice.com.
cust.b.hostedemail.com. 5 NS ns2.mdnsservice.com.
cust.b.hostedemail.com. 5 NS ns1.mdnsservice.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0