Why have people got so upset over the whole client/server side filtering issue, for me it's important to have server side filtering that way when I go between webmail and a desktop client I have consistency, I only have to setup filtering rules once, rather then for each client, etc etc etc, and since you need the webmail client running on the server it seems pointless to have redundant code on both the MTA and the MUA, especially considering that a lot of MTAs already support sieve filtering, why re-invent the wheel?
As I posted in another thread I recently switched from Cyrus to Dovecot, Cyrus comes with Sieve code built in as does Dovecot, but Cyrus has a management interface that listens on port 2000, unfortunately Dovecot doesn't yet include the management daemon yet, but luckily there is a python daemon someone wrote that does.
All up took me 5 mins or so to get sieve filtering working on the server, the bigger issue was no interface like avelsieve in roundcube, time permitting I hope to rectify this.
What I did find and what works ok for now is SmartSieve (
http://smartsieve.sourceforge.net) but it lacks the integration with the webmail client and the pointy clicky goodness of avelsieve, the downside for SmartSieve is it requires the imap extensions for php, but 5 mins with a hacksaw and that requirement went bye-bye. The reason it needed the imap extension was so it could get a list of folder names for when you point mail directly to some other folder then Inbox.
In any case this is only one of many php sieve clients, most if not all are GPL, so all that needs to be done is for someone to hack existing code into the RC interface in a similar way that avelsieve works for squirrelmail.