mailfilter.hostnet.nl has no CNAME records.
Question and response
Question
dig mailfilter.hostnet.nl. NS
Answer
The response has no answer records.
Authority
hostnet.nl. 300 SOA ns01.authdns.hostnet.nl. hostmaster.hostnet.nl. 2026022001 1800 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0