Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-out.amazonses.com. NS
Answer
smtp-out.amazonses.com. 5 NS ns-1872.awsdns-42.co.uk.
smtp-out.amazonses.com. 5 NS ns-1208.awsdns-23.org.
smtp-out.amazonses.com. 5 NS ns-298.awsdns-37.com.
smtp-out.amazonses.com. 5 NS ns-919.awsdns-50.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0