mx1.mailfilter.no has no CNAME records.
Question and response
Question
dig mx1.mailfilter.no. NS
Answer
The response has no answer records.
Authority
mailfilter.no. 3533 SOA cert-dns.itpays.net. hostmaster.itpays.net. 2022011204 10800 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0