Hi there I wonder, can email users log somehow into ISPC and add/alter their mail filtering rules? As far as I can see only "clients" can login and set the filtering for the individual email users.
A mailuser login is already available in svn and will be released with ISPConfig 3.0.5. Cuurrently, mailusers can alter their filter rules in their webmail client (roundcube or squirrelmail) when the ispconfig plugins are installed.