Apex domains cannot have CNAME records.
Question and response
Question
dig inbound-smtp.us-east-2.amazonaws.com. NS
Answer
inbound-smtp.us-east-2.amazonaws.com. 5 NS ns-1755.awsdns-27.co.uk.
inbound-smtp.us-east-2.amazonaws.com. 5 NS ns-402.awsdns-50.com.
inbound-smtp.us-east-2.amazonaws.com. 5 NS ns-638.awsdns-15.net.
inbound-smtp.us-east-2.amazonaws.com. 5 NS ns-1270.awsdns-30.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0