Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-out.eu-west-2.amazonses.com. NS
Answer
smtp-out.eu-west-2.amazonses.com. 5 NS ns-338.awsdns-42.com.
smtp-out.eu-west-2.amazonses.com. 5 NS ns-1222.awsdns-24.org.
smtp-out.eu-west-2.amazonses.com. 5 NS ns-524.awsdns-01.net.
smtp-out.eu-west-2.amazonses.com. 5 NS ns-2039.awsdns-62.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0