Mail system enhancements - recommendations?

[bronto]

I currently have a mail system set up on my server that is serving me 
well, but I want to add features to it that my other user/clients 
want.  Currently the system is Postfix/Courier-imap/MySQL based, with 
the mail users all virtual.  I like that combination and don't want 
to change it.  The server is running YellowDog Linux (PPC 
architecture), so some of the solutions may require source code to 
compile for this system.

What I want to add:

* Spam filtering
* virus filtering
* user configurable message filtering to appropriate mailboxes
* web mail
* autoresponders

I've browsed through the postfix site and looked at a few of the 
options on the Addon Software page.  The mail filtering options 
appear to best be served by either procmail or maildrop, and I've 
installed squirrelmail and looked at IMP for the web mail.  There 
appears to be many options.  The concern I have is that of 
"chemistry".  Squirrelmail looks good, but a lot of the cool plugins 
seem to assume that the mail user has a specific unix user account, 
including the procmail one.  Can this work with virtual users?  Is 
there a maildrop interface for squirrelmail or IMP (or some other 
one?).  Of all these options, which are going to work *together* the 
best?

I would appreciate hearing the the wisdom of those that have done 
something like this before.

Rob