mx.drmarkgreene.com has no CNAME records.
Question and response
Question
dig mx.drmarkgreene.com. NS
Answer
The response has no answer records.
Authority
drmarkgreene.com. 3475 SOA dns.drmarkgreene.com. hostmaster.drmarkgreene.com. 1153593874 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0