Apex domains cannot have CNAME records.
Question and response
Question
dig inbound-smtp.eu-west-1.amazonaws.com. NS
Answer
inbound-smtp.eu-west-1.amazonaws.com. 5 NS ns-1404.awsdns-47.org.
inbound-smtp.eu-west-1.amazonaws.com. 5 NS ns-783.awsdns-33.net.
inbound-smtp.eu-west-1.amazonaws.com. 5 NS ns-300.awsdns-37.com.
inbound-smtp.eu-west-1.amazonaws.com. 5 NS ns-1934.awsdns-49.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0