Apex domains cannot have CNAME records.
Question and response
Question
dig us-west-2.mail-perimeter.amazon.com. NS
Answer
us-west-2.mail-perimeter.amazon.com. 5 NS ns-1975.awsdns-54.co.uk.
us-west-2.mail-perimeter.amazon.com. 5 NS ns-1096.awsdns-09.org.
us-west-2.mail-perimeter.amazon.com. 5 NS ns-487.awsdns-60.com.
us-west-2.mail-perimeter.amazon.com. 5 NS ns-945.awsdns-54.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0