mx.myourtheir.com has no NS records.
NS records for this zone are defined at myourtheir.com.
NS records for this zone are defined at myourtheir.com.
Question and response
Question
dig mx.myourtheir.com. NS
Answer
The response has no answer records.
Authority
myourtheir.com. 3600 SOA dns.myourtheir.com. hostmaster.myourtheir.com. 1498506046 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0