Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-out.ap-south-1.amazonses.com. NS
Answer
smtp-out.ap-south-1.amazonses.com. 5 NS ns-737.awsdns-28.net.
smtp-out.ap-south-1.amazonses.com. 5 NS ns-88.awsdns-11.com.
smtp-out.ap-south-1.amazonses.com. 5 NS ns-1965.awsdns-53.co.uk.
smtp-out.ap-south-1.amazonses.com. 5 NS ns-1185.awsdns-20.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0