Apex domains cannot have CNAME records.
Question and response
Question
dig mail.pcextreme.nl. NS
Answer
mail.pcextreme.nl. 5 NS ns053.auroradns.info.
mail.pcextreme.nl. 5 NS ns052.auroradns.nl.
mail.pcextreme.nl. 5 NS ns051.auroradns.eu.
Authority
The response has no authority records.
Additional
ns051.auroradns.eu. 1 A 185.95.31.118
ns052.auroradns.nl. 1 A 31.25.98.35
ns053.auroradns.info. 1 A 195.8.208.24
ns051.auroradns.eu. 1 AAAA 2a00:f10:402:781:1c00:d0ff:fe00:2521
ns052.auroradns.nl. 1 AAAA 2a05:1500:702:0:1c00:a2ff:fe00:1f
ns053.auroradns.info. 1 AAAA 2a05:1500:fe00:0:0:c3ff:fe08:d018
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0