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.