Apex domains cannot have CNAME records.
Question and response
Question
dig sealmail.co. NS
Answer
sealmail.co. 5 NS nameserver02.mi.com.co.
sealmail.co. 5 NS nameserver03.mi.com.co.
sealmail.co. 5 NS nameserver04.mi.com.co.
sealmail.co. 5 NS nameserver01.mi.com.co.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0