Apex domains cannot have CNAME records.
Question and response
Question
dig smtpeter.com. NS
Answer
smtpeter.com. 5 NS dns1.proxi.nl.
smtpeter.com. 5 NS dns2.proxi.nl.
Authority
The response has no authority records.
Additional
dns1.proxi.nl. 5 A 89.149.192.4
dns2.proxi.nl. 5 A 89.149.192.56
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0