mail.sieci.net.pl has no CNAME records.
Question and response
Question
dig mail.sieci.net.pl. NS
Answer
The response has no answer records.
Authority
sieci.net.pl. 3314 SOA dns1.obslugasieci.pl. pczerw.sieci.net.pl. 2025122801 7200 540 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0