i'm currently testing ispconfig and will be implementing this next weekend. I use roundcube as well and want to work with sieve filters. The filters set up in ISPconfig didn't work with the dovecot 1.2.9 on the ubuntu 10.04 system although i changed the query in dovecot.
The plugin "sieverules" which communicates to the managasieve plugin in dovecot works good, but it does check if disablesieve is 'n' in the mail_user table by this command specified in the dovecot-sql.conf
password_query = SELECT password FROM mail_user WHERE email = '%u' AND disable%Ls = 'n'
And you want to keep the last part of this code to check if pop or imap is disabled...
My suggestion for the next verison of ispconfig would be to implement the field disablesieve with the default value 'n' in the table mail_user. The command below can be used.
ALTER TABLE `mail_user` ADD `disablesieve` ENUM( 'n', 'y' ) NOT NULL DEFAULT 'n'
this would be great for me and probably others as well. And you would be able to select which mail users are allowed to create sieve filters.
Cheers, Bart Dorlandt