Apex domains cannot have CNAME records.
Question and response
Question
dig seppmaildev.com. NS
Answer
seppmaildev.com. 5 NS ns2.itmuller.com.
seppmaildev.com. 5 NS ns1.itmuller.com.
seppmaildev.com. 5 NS ns1.itmnetx.com.
seppmaildev.com. 5 NS ns2.itmnetx.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0