mx.plugmeon.com has no CNAME records.
Question and response
Question
dig mx.plugmeon.com. NS
Answer
The response has no answer records.
Authority
plugmeon.com. 3558 SOA dns.plugmeon.com. hostmaster.plugmeon.com. 1527380378 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0