mx5.mssm.pl has no CNAME records.
Question and response
Question
dig mx5.mssm.pl. NS
Answer
The response has no answer records.
Authority
mssm.pl. 10800 SOA ns1.mssm.pl. root.mssm.pl. 2025020701 1080 3600 60480 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0