how bout a "delete all emails before XX/XX/XXXX date" option, or a "delete older than XX days" option?
Not sure how you could implement this, but I'd like it too, if it could be applied on a folder-by-folder basis. Or even if the only two folders that it could do were Trash and Sent.
there is EXPIRE plugin fo dovecot to do this:
http://wiki.dovecot.org/Plugins/Expire
BUT! it don't have a very important feat: KEEP marked important messages!
Thunderbird or TheBat mail clients have it.
Whithout this option EXPIRE plugin for dovecot is much useless in most situations.