per user server side filtering for virtual users

[MrRagga]

Hi,
I am wondering how to set up per user server side filtering as easy as possible. As far as i understand I can only use a procmailrc for local users, since virtual_transport is set to virtual. The server side filtering should be possible for every email account, so that i can filter every message to the appropriate maildir folder.
So what is the easiest solution to enable this? It doesn't matter if i use maildrop, procmail or whatever. I am just interested in that kind of solution without modifying too much.

Cheers Jonas