mx.musvc.com has no MX records.
Question and response
Question
dig mx.musvc.com. MX
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