mx.mailfiltering.com has no CNAME records.
Question and response
Question
dig mx.mailfiltering.com. NS
Answer
The response has no answer records.
Authority
mailfiltering.com. 100 SOA dns1.vps.net. root.mailfiltering.com. 2025111802 7200 600 1728000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0