mx1.netinstal.pl has no CNAME records.
Question and response
Question
dig mx1.netinstal.pl. NS
Answer
The response has no answer records.
Authority
netinstal.pl. 1478 SOA ns1.netinstal.pl. root.netinstal.pl. 2025121601 3600 3600 60480 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0