Apex domains cannot have CNAME records.
Question and response
Question
dig seppmaildemo.com. NS
Answer
seppmaildemo.com. 5 NS ns.seppmail.cloud.
seppmaildemo.com. 5 NS ns.seppmail.ch.
seppmaildemo.com. 5 NS ns.seppmail.io.
Authority
The response has no authority records.
Additional
ns.seppmail.ch. 5 A 194.58.198.32
ns.seppmail.io. 5 A 194.58.192.32
ns.seppmail.cloud. 5 A 185.42.137.98
ns.seppmail.ch. 5 AAAA 2a01:3f1:3032:0:0:0:0:53
ns.seppmail.io. 5 AAAA 2a01:3f1:32:0:0:0:0:53
ns.seppmail.cloud. 5 AAAA 2a01:3f0:400:0:0:0:0:32
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0