Apex domains cannot have CNAME records.
Question and response
Question
dig smtpcorp.com. NS
Answer
smtpcorp.com. 4 NS augustus.ns.cloudflare.com.
smtpcorp.com. 4 NS ada.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0