Apex domains cannot have CNAME records.
Question and response
Question
dig smtp24.com. NS
Answer
smtp24.com. 3 NS jim.ns.cloudflare.com.
smtp24.com. 3 NS amy.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0