Apex domains cannot have CNAME records.
Question and response
Question
dig rs136-realsender.com. NS
Answer
rs136-realsender.com. 4 NS ns-1891.awsdns-44.co.uk.
rs136-realsender.com. 4 NS ns-47.awsdns-05.com.
rs136-realsender.com. 4 NS ns-920.awsdns-51.net.
rs136-realsender.com. 4 NS ns-1199.awsdns-21.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0