mailserver.cpserver.net has no CNAME records.
Question and response
Question
dig mailserver.cpserver.net. NS
Answer
The response has no answer records.
Authority
cpserver.net. 4940 SOA ns01.cpserver.net. hello.cpserver.net. 2026022500 3600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0