mx20.pbs.edu.pl has no CNAME records.
Question and response
Question
dig mx20.pbs.edu.pl. NS
Answer
The response has no answer records.
Authority
pbs.edu.pl. 843 SOA sun1.utp.edu.pl. please_set_email.absolutely.nowhere. 1341 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0