Apex domains cannot have CNAME records.
Question and response
Question
dig amazon-smtp.amazon.com. NS
Answer
amazon-smtp.amazon.com. 4 NS ns-330.awsdns-41.com.
amazon-smtp.amazon.com. 4 NS ns-1471.awsdns-55.org.
amazon-smtp.amazon.com. 4 NS ns-827.awsdns-39.net.
amazon-smtp.amazon.com. 4 NS ns-1565.awsdns-03.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0