mx.b.hostedemail.com has no NS records.
NS records for this zone are defined at b.hostedemail.com.
NS records for this zone are defined at b.hostedemail.com.
Question and response
Question
dig mx.b.hostedemail.com. NS
Answer
The response has no answer records.
Authority
b.hostedemail.com. 300 SOA ns1.mdnsservice.com. hostmaster.mdnsservice.com. 957775201 10001 7200 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0