Apex domains cannot have CNAME records.
Question and response
Question
dig mailplus.nl. NS
Answer
mailplus.nl. 5 NS rory.ns.cloudflare.com.
mailplus.nl. 5 NS brianna.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0