CiviMail Processor: Delete old processed emails

CiviMail Processor puts the processed emails in a 'backup' Maildir folder. This may cause problems when used to process large amounts of emails: http://forum.civicrm.org/?topic=7908

One solution is to move the processed emails to a $year/$month/$day subfolder structure.

The relevant cronjob should also check for old (older than two months?) emails there and delete them first, before polling the source(s).