No,i didn't change the hostname in /etc/mail so one problem's fixed now.
hostname -f gives me ok.mbps.pl so it's ok.
Now after reboot my administrator's email account can recieve mail but after sending a letter to another account on the system i get an answer again but with a different error like this:"
This is the Postfix program at host ok.mbps.pl.
I'm sorry to have to inform you that your message could not be
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to <postmaster>
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The Postfix program
<my_user_address>: maildir delivery failed: create
Can this problem be fixed automatically by ISPConfig or should I have to do it myself ?