Apex domains cannot have CNAME records.
Question and response
Question
dig rs701-realsender.com. NS
Answer
rs701-realsender.com. 5 NS ns-22.awsdns-02.com.
rs701-realsender.com. 5 NS ns-1596.awsdns-07.co.uk.
rs701-realsender.com. 5 NS ns-1398.awsdns-46.org.
rs701-realsender.com. 5 NS ns-763.awsdns-31.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0