Apex domains cannot have CNAME records.
Question and response
Question
dig simplemailservice.eu. NS
Answer
simplemailservice.eu. 5 NS ns2.premiumdns.zone.
simplemailservice.eu. 5 NS ns1.premiumdns.zone.
Authority
The response has no authority records.
Additional
ns1.premiumdns.zone. 5 A 192.174.68.15
ns2.premiumdns.zone. 5 A 176.97.158.15
ns1.premiumdns.zone. 5 AAAA 2001:67c:1bc:0:0:0:0:15
ns2.premiumdns.zone. 5 AAAA 2001:67c:10b8:0:0:0:0:15
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0