Apex domains cannot have CNAME records.
Question and response
Question
dig eu-central-1.mail-perimeter.amazon.com. NS
Answer
eu-central-1.mail-perimeter.amazon.com. 5 NS ns-1802.awsdns-33.co.uk.
eu-central-1.mail-perimeter.amazon.com. 5 NS ns-518.awsdns-00.net.
eu-central-1.mail-perimeter.amazon.com. 5 NS ns-117.awsdns-14.com.
eu-central-1.mail-perimeter.amazon.com. 5 NS ns-1212.awsdns-23.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0