mx.musvc.com has no NS records.
NS records for this zone are defined at musvc.com.
NS records for this zone are defined at musvc.com.
Question and response
Question
dig mx.musvc.com. NS
Answer
The response has no answer records.
Authority
musvc.com. 7200 SOA ns-47.awsdns-05.com. domain.mailup.com. 2012052263 3600 3600 1209600 7200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0