mail.mxfilter.net has no CNAME records.
Question and response
Question
dig mail.mxfilter.net. NS
Answer
The response has no answer records.
Authority
mxfilter.net. 9872 SOA ns1.private-nameserver.net. admin.namecrane.net. 2025100902 3600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0