Hi ! I have started installing ISPConfig and found one interesting thing: it creates UNIX user for each Site -> User&E-Mail (I am was thinking before that ISPConfig would use authentication via MySQL database). So, If, for example, I will have 500 e-mails, I will end up with extra 500 UNIX users?
If you want virtual users, you will have to use the upcoming ISPConfig 3, have a look at the development forum to get one of the alpha versions. But 500 system users are no problem on a linux system.