mail.insite.pl has no MX records.
Question and response
Question
dig mail.insite.pl. MX
Answer
The response has no answer records.
Authority
insite.pl. 3600 SOA insite.pl. dns.insite.pl. 2007011573 7200 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0