Apex domains cannot have CNAME records.
Question and response
Question
dig sudumail.com. NS
Answer
sudumail.com. 5 NS ens12.bigwww.com.
sudumail.com. 5 NS ens11.bigwww.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0