mail.ispsystem.com has no CNAME records.
Question and response
Question
dig mail.ispsystem.com. NS
Answer
The response has no answer records.
Authority
ispsystem.com. 238 SOA dns1.ispsystem.net. admin.ispsystem.com. 2026022002 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0