Automatic cleaning of the Junk folder for Dovecot
There is same feature request for Courier:
http://bugtracker.ispconfig.org/index.php?do=details&action=details.addvote&task_id=2425
but I use Dovecot.
Dovecot use Expire plugin for automate cleaning of old mails.
For Dovecot 1.x - http://wiki1.dovecot.org/Plugins/Expire For Dovecot 2.x - http://wiki2.dovecot.org/Plugins/Expire
Option for automate cleaning should be set globaly and should be changable for each mailbox by its owner too.