Hi everybody,
This discussion is mostly about message filters. This was discussed over and over at this list: http://lists.roundcube.net/mail-archive/roundcube.dev/2006/04/38/ http://lists.roundcube.net/mail-archive/roundcube.dev/2006/07/73/
There's even a feature request pending for that: http://trac.roundcube.net/trac.cgi/ticket/1326320
All I can say is that I don't like to implement Yet Another Message Filter but rely on server side filters. This is more efficient and - as Thomas mentioned - keeps your Inbox from becoming a war zone if you once check your mail with a different client that doesn't have all the rules configured.
Let's keep RoundCube a mail CLIENT and focus on the creation of a plugin API which can be used to build all sorts of admin panels for procmail, Sieve, and whatever.
~Thomas