mx.sell-hub.com has no CNAME records.
Question and response
Question
dig mx.sell-hub.com. NS
Answer
The response has no answer records.
Authority
sell-hub.com. 3511 SOA dns.sell-hub.com. hostmaster.sell-hub.com. 1916673369 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0