Apex domains cannot have CNAME records.
Question and response
Question
dig us-east-1.outbound.mail-perimeter.amazon.com. NS
Answer
us-east-1.outbound.mail-perimeter.amazon.com. 5 NS ns-1205.awsdns-22.org.
us-east-1.outbound.mail-perimeter.amazon.com. 5 NS ns-1829.awsdns-36.co.uk.
us-east-1.outbound.mail-perimeter.amazon.com. 5 NS ns-423.awsdns-52.com.
us-east-1.outbound.mail-perimeter.amazon.com. 5 NS ns-754.awsdns-30.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0