mx.spamfilter.io has no CNAME records.
Question and response
Question
dig mx.spamfilter.io. NS
Answer
The response has no answer records.
Authority
spamfilter.io. 63 SOA ns1.wannafind.dk. hostmaster.wannafind.dk. 2025112702 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0