Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-out.us-east-2.amazonses.com. NS
Answer
smtp-out.us-east-2.amazonses.com. 5 NS ns-1892.awsdns-44.co.uk.
smtp-out.us-east-2.amazonses.com. 5 NS ns-1346.awsdns-40.org.
smtp-out.us-east-2.amazonses.com. 5 NS ns-414.awsdns-51.com.
smtp-out.us-east-2.amazonses.com. 5 NS ns-685.awsdns-21.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0