Apex domains cannot have CNAME records.
Question and response
Question
dig spf.realsender.com. NS
Answer
spf.realsender.com. 4 NS ns-1640.awsdns-13.co.uk.
spf.realsender.com. 4 NS ns-1257.awsdns-29.org.
spf.realsender.com. 4 NS ns-412.awsdns-51.com.
spf.realsender.com. 4 NS ns-854.awsdns-42.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0