mail01.omdc.pl has no CNAME records.
Question and response
Question
dig mail01.omdc.pl. NS
Answer
The response has no answer records.
Authority
omdc.pl. 1013 SOA ns5.omdc.pl. admin.omdc.pl. 2011042269 10800 5400 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0