Apex domains cannot have CNAME records.
Question and response
Question
dig email-smtp.us-east-1.amazonaws.com. NS
Answer
email-smtp.us-east-1.amazonaws.com. 5 NS ns-757.awsdns-30.net.
email-smtp.us-east-1.amazonaws.com. 5 NS ns-242.awsdns-30.com.
email-smtp.us-east-1.amazonaws.com. 5 NS ns-1934.awsdns-49.co.uk.
email-smtp.us-east-1.amazonaws.com. 5 NS ns-1488.awsdns-58.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0