mail.skydata.pl has no CNAME records.
Question and response
Question
dig mail.skydata.pl. NS
Answer
The response has no answer records.
Authority
skydata.pl. 3174 SOA ns1.pro-linuxpl.com. hostmaster.skydata.pl. 2023110600 14400 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0