Mail.pubnix.net has no CNAME records.
Question and response
Question
dig Mail.pubnix.net. NS
Answer
The response has no answer records.
Authority
pubnix.net. 70993 SOA ns1.pubnix.net. root.pubnix.net. 2025085127 3600 900 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0