spamfilter.nexthost.nl has no CNAME records.
Question and response
Question
dig spamfilter.nexthost.nl. NS
Answer
The response has no answer records.
Authority
nexthost.nl. 1791 SOA ns01.nexthost.nl. hostmaster.nexthost.nl. 2025100800 3600 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0