Apex domains cannot have CNAME records.
Question and response
Question
dig smtp.com. NS
Answer
smtp.com. 5 NS ns-5.awsdns-00.com.
smtp.com. 5 NS ns-1304.awsdns-35.org.
smtp.com. 5 NS ns-735.awsdns-27.net.
smtp.com. 5 NS ns-1999.awsdns-57.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0