mx.sldc.pl has no CNAME records.
Question and response
Question
dig mx.sldc.pl. NS
Answer
The response has no answer records.
Authority
sldc.pl. 99 SOA dns1.slaskdatacenter.pl. admin.slaskdatacenter.pl. 2023051520 100 100 100 100
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0