Apex domains cannot have CNAME records.
Question and response
Question
dig onlinemail.io. NS
Answer
onlinemail.io. 5 NS ns1.curanet.dk.
onlinemail.io. 5 NS ns2.curanet.dk.
Authority
The response has no authority records.
Additional
ns1.curanet.dk. 2 A 185.25.141.12
ns2.curanet.dk. 2 A 91.197.248.75
ns1.curanet.dk. 2 AAAA 2a02:2339:4006:108:0:0:0:123
ns2.curanet.dk. 2 AAAA 2a02:2339:1:10f:0:0:123:123
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0