mail.polko.net has no CNAME records.
Question and response
Question
dig mail.polko.net. NS
Answer
The response has no answer records.
Authority
polko.net. 1918 SOA ns1.polko.net. hostmaster.polko.net. 2026011503 86400 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0