Dovecot CMUSieve plugin provides essential mail filtering capabilities - based on header flags or regex, vacation auto reply, notifications, etcetera. Please see the http://wiki.dovecot.org/LDA/Sieve for additional info and configuration help. To use dovecot-cmusieve effectively, you will have to patch dovecot to include managesieve support. Without managesieve, users would need have at least FTP or SCP access to server to upload their sieve scripts and you would not have anyway to check for errors in sieve scripts. Managesieve provides easy access and script checking support so it is highly desirable. To create/manage sieve scripts for mail filtering, you can use kmail (which has native sieve support) or use thunderbird with sieve extension. The sieve extension for thunderbird is still under development. You can find more info on that at http://sieve.mozdev.org/ This requires dovecot.