Actualy I finished with your tutorial http://www.howtoforge.com/virtual_po..._quota_courier
and I have YOUR server.I had problems but with YOUR answers I solved every problem step by step..
So is it YOUR server(I started with other tutorials but finally...I am here)
The most importan part is to have a mail server with users limitations(smtp restrictions ).I tryed few months with qmail but I must write the authentication package there so I say better to migrate to postfix.
Thank you for your promtitude.