Apex domains cannot have CNAME records.
Question and response
Question
dig inbound-smtp.us-west-2.amazonaws.com. NS
Answer
inbound-smtp.us-west-2.amazonaws.com. 5 NS ns-453.awsdns-56.com.
inbound-smtp.us-west-2.amazonaws.com. 5 NS ns-1635.awsdns-12.co.uk.
inbound-smtp.us-west-2.amazonaws.com. 5 NS ns-1152.awsdns-16.org.
inbound-smtp.us-west-2.amazonaws.com. 5 NS ns-585.awsdns-09.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0