Apex domains cannot have CNAME records.
Question and response
Question
dig us-east-1.mail-perimeter.amazon.com. NS
Answer
us-east-1.mail-perimeter.amazon.com. 5 NS ns-1397.awsdns-46.org.
us-east-1.mail-perimeter.amazon.com. 5 NS ns-1820.awsdns-35.co.uk.
us-east-1.mail-perimeter.amazon.com. 5 NS ns-226.awsdns-28.com.
us-east-1.mail-perimeter.amazon.com. 5 NS ns-750.awsdns-29.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0