Apex domains cannot have CNAME records.
Question and response
Question
dig rspf.letsdmarc.com. NS
Answer
rspf.letsdmarc.com. 3 NS ns1.letsdmarc.com.
rspf.letsdmarc.com. 3 NS ns3.letsdmarc.com.
rspf.letsdmarc.com. 3 NS ns2.letsdmarc.com.
rspf.letsdmarc.com. 3 NS ns4.letsdmarc.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0