mail.zonnet.net has no CNAME records.
Question and response
Question
dig mail.zonnet.net. NS
Answer
The response has no answer records.
Authority
zonnet.net. 10648 SOA ns.zonnet.nl. hostmaster.zonnet.nl. 2020083100 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0