Apex domains cannot have CNAME records.
Question and response
Question
dig outbound.mail-perimeter.amazon.com. NS
Answer
outbound.mail-perimeter.amazon.com. 5 NS ns-864.awsdns-44.net.
outbound.mail-perimeter.amazon.com. 5 NS ns-1512.awsdns-61.org.
outbound.mail-perimeter.amazon.com. 5 NS ns-1924.awsdns-48.co.uk.
outbound.mail-perimeter.amazon.com. 5 NS ns-101.awsdns-12.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0