Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-out.ap-southeast-2.amazonses.com. NS
Answer
smtp-out.ap-southeast-2.amazonses.com. 5 NS ns-1553.awsdns-02.co.uk.
smtp-out.ap-southeast-2.amazonses.com. 5 NS ns-493.awsdns-61.com.
smtp-out.ap-southeast-2.amazonses.com. 5 NS ns-902.awsdns-48.net.
smtp-out.ap-southeast-2.amazonses.com. 5 NS ns-1221.awsdns-24.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0