mx.mail.ispapi.net has no CNAME records.
Question and response
Question
dig mx.mail.ispapi.net. NS
Answer
The response has no answer records.
Authority
mail.ispapi.net. 3600 SOA ns1.ispapi.net. mail.ispapi.net.mail.ispapi.net. 2022022800 86400 7200 3600000 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0