Apex domains cannot have CNAME records.
Question and response
Question
dig rdns.jetsmtp.net. NS
Answer
rdns.jetsmtp.net. 4 NS ns2.jetsmtp.net.
rdns.jetsmtp.net. 4 NS ns1.jetsmtp.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0