Apex domains cannot have CNAME records.
Question and response
Question
dig mail-perimeter.amazon.com. NS
Answer
mail-perimeter.amazon.com. 5 NS ns-1036.awsdns-01.org.
mail-perimeter.amazon.com. 5 NS ns-204.awsdns-25.com.
mail-perimeter.amazon.com. 5 NS ns-921.awsdns-51.net.
mail-perimeter.amazon.com. 5 NS ns-1606.awsdns-08.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0