mx1.admin.net.pl has no CNAME records.
Question and response
Question
dig mx1.admin.net.pl. NS
Answer
The response has no answer records.
Authority
admin.net.pl. 930 SOA ns1.admin.net.pl. admin.admin.net.pl. 2026130001 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0