mail.mostrowski.pl has no CNAME records.
Question and response
Question
dig mail.mostrowski.pl. NS
Answer
The response has no answer records.
Authority
mostrowski.pl. 3395 SOA ns1.hostido.net.pl. hostmaster.mostrowski.pl. 2026012401 86400 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0