What's in /etc/postfix/main.cf, /etc/postfix/local-host-names, and /etc/postfix/virtusertable?
As far as the authentication issue is concerned, None of the site administrators can login to their control panel. I can login as the server administrator but not as the site administrator. Everyone can login as a mailuser with no problem too.
Also, just a note, I have never been able to ftp in to any site unless I use SFTP. This isn't much of a problem for me but it is a real problem for my customers because they don't even know what SFTP is.
Please make sure you use the correct usernames. Please don't mix up ISPConfig users and system users!