Apex domains cannot have CNAME records.
Question and response
Question
dig gate.seppmail.cloud. NS
Answer
gate.seppmail.cloud. 1 NS ns.seppmail.ch.
gate.seppmail.cloud. 1 NS ns.seppmail.io.
gate.seppmail.cloud. 1 NS ns.seppmail.cloud.
Authority
The response has no authority records.
Additional
ns.seppmail.ch. 1 A 194.58.198.32
ns.seppmail.io. 1 A 194.58.192.32
ns.seppmail.cloud. 1 A 185.42.137.98
ns.seppmail.ch. 1 AAAA 2a01:3f1:3032:0:0:0:0:53
ns.seppmail.io. 1 AAAA 2a01:3f1:32:0:0:0:0:53
ns.seppmail.cloud. 1 AAAA 2a01:3f0:400:0:0:0:0:32
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0