MassTransit can set up to automatically clean up Mailbox folders using the FileExpirationAndDeletion script and Actions. It free to customers current on support and is available for download online.
The full directions for set up are in the script, but basically you will set up an Action that is set to run as a periodic event. The script can clear out the To Send, To Be Forwarded, Sent and Received folders or any combination of the Mailbox folders. It can also be set to send a reminder notification to Web Client users that have not picked up their files within a configured interval and that if they don’t they will be deleted at another configured interval. This notice only goes out once. The script will delete the files from the file system as well as the file records from the database. If you choose to enable this for all of the folders you should disable the internal file record cleanup for MassTransit in the MassTransit Administrator on the Setup -> Special tab. If you only choose to do a subset of these folders, leave this setting enabled, however, make sure that you set it to a higher number than any of the time intervals you set for the folders in the script or else the script will not find the file records to be able to delete files and will defeat the purpose of the script.
For example, if in the script you set the To Send folder to delete files and file records older than 10 days old, and you have the internal MassTransit file cleanup set to delete files records older than 5 days old, there will never be any file records older than 10 days old for the script to find.