Apex domains cannot have CNAME records.
Question and response
Question
dig inbound-smtp.eu-west-2.amazonaws.com. NS
Answer
inbound-smtp.eu-west-2.amazonaws.com. 5 NS ns-298.awsdns-37.com.
inbound-smtp.eu-west-2.amazonaws.com. 5 NS ns-655.awsdns-17.net.
inbound-smtp.eu-west-2.amazonaws.com. 5 NS ns-1452.awsdns-53.org.
inbound-smtp.eu-west-2.amazonaws.com. 5 NS ns-1852.awsdns-39.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0