mx.drilldocter.com has no CNAME records.
Question and response
Question
dig mx.drilldocter.com. NS
Answer
The response has no answer records.
Authority
drilldocter.com. 3523 SOA dns.drilldocter.com. hostmaster.drilldocter.com. 1533881471 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0