mail.debnet.pl has no CNAME records.
Question and response
Question
dig mail.debnet.pl. NS
Answer
The response has no answer records.
Authority
debnet.pl. 2111 SOA dns1.debnet.pl. tech.debnet.pl. 2025041602 3600 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0