Apex domains cannot have CNAME records.
Question and response
Question
dig inbound-smtp.us-east-1.amazonaws.com. NS
Answer
inbound-smtp.us-east-1.amazonaws.com. 5 NS ns-1827.awsdns-36.co.uk.
inbound-smtp.us-east-1.amazonaws.com. 5 NS ns-778.awsdns-33.net.
inbound-smtp.us-east-1.amazonaws.com. 5 NS ns-152.awsdns-19.com.
inbound-smtp.us-east-1.amazonaws.com. 5 NS ns-1450.awsdns-53.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0