Apex domains cannot have CNAME records.
Question and response
Question
dig mailport.nl. NS
Answer
mailport.nl. 5 NS intention.ns02.nl.
mailport.nl. 5 NS intention.ns01.nl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0