Here's a quick walkthrough on how to use the Exchange Mailbox Manager in Exchange 2000 or 2003.
SofTouch recommends at least configuring the "Deleted Items" folder for "Delete Immediately" with the standard settings of 30 days or 1024KB. This will allow the policy to run every weekend (either Friday or Saturday night) but only delete items that are older than 30 days, or larger than 1MB.
