Apex domains cannot have CNAME records.
Question and response
Question
dig mail.ispapi.net. NS
Answer
mail.ispapi.net. 5 NS ns2.ispapi.net.
mail.ispapi.net. 5 NS ns1.ispapi.net.
mail.ispapi.net. 5 NS ns3.ispapi.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0