Apex domains cannot have CNAME records.
Question and response
Question
dig smtp18.com. NS
Answer
smtp18.com. 1 NS amy.ns.cloudflare.com.
smtp18.com. 1 NS jim.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0