Spam Rule is a plugin for SquirrelMail that lets you select from different groups of email filters via a config file maintained through FTP.  It is written specifically for situations where a user can select from various filters, individually or in predefined groups, and subscribe to them by modifing lines in a procmail or maildrop config file in their home directory.  This way the user can have his mail filtered at message delivery time by the email system rather than at reading time by SquirrelMail, which could really slow things down.