Apex domains cannot have CNAME records.
Question and response
Question
dig rs501-realsender.com. NS
Answer
rs501-realsender.com. 5 NS ns-1766.awsdns-28.co.uk.
rs501-realsender.com. 5 NS ns-1403.awsdns-47.org.
rs501-realsender.com. 5 NS ns-294.awsdns-36.com.
rs501-realsender.com. 5 NS ns-656.awsdns-18.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0