Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-out.eu-west-3.amazonses.com. NS
Answer
smtp-out.eu-west-3.amazonses.com. 5 NS ns-1226.awsdns-25.org.
smtp-out.eu-west-3.amazonses.com. 5 NS ns-1882.awsdns-43.co.uk.
smtp-out.eu-west-3.amazonses.com. 5 NS ns-250.awsdns-31.com.
smtp-out.eu-west-3.amazonses.com. 5 NS ns-959.awsdns-55.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0