After finally finishing all the tutorials and succeeded debugging my own mistakes, I have a nice server running, thanks to Falko's HOWTO.
(The Virtual Users And Domains With Postfix, Courier And MySQL (+ SMTP-AUTH, Quota, SpamAssassin, ClamAV)
The only little problem I have found now is that I can't get the system messages that are by default delivered to root, to be forwarded to another account so I can check them out.
Somewhere in the setup process, I was asked to input whom will get root mails.
at the end of the process, all my server is receiving mail only to the virtual users, and it seems that the /etc/aliases file doesn't affect the local mail transfers, because there I see it says that root : firstname.lastname@example.org
should get root mail, but it doesn't!
How can I fix it up?