Apex domains cannot have CNAME records.
Question and response
Question
dig rs700-realsender.com. NS
Answer
rs700-realsender.com. 5 NS ns-1207.awsdns-22.org.
rs700-realsender.com. 5 NS ns-1754.awsdns-27.co.uk.
rs700-realsender.com. 5 NS ns-223.awsdns-27.com.
rs700-realsender.com. 5 NS ns-911.awsdns-49.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0