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At present, the Auto-Clean mechanism appears to base its message deletion decision on the date the message was sent/received (i.e. the message date). This works well for Inbox, Sent Items, and Junk E-mail; folders in which messages are usually placed at the time they are sent/received. The Deleted Items folder, however, is a different scenario.
Messages could be, and often are, deleted (moved to Deleted Items) long after they were originally sent/received. With the current Auto-Clean mechanism, these messages are essentially auto-cleaned immediately (the next time the process runs), rather than waiting the specified period of time as one might expect. For example, if I'm cleaning out my Inbox and delete a message that I received 6 months ago, that message would be auto-cleaned from the Deleted Items folder later that night, rather than waiting the period of time specified in the Auto-Clean setup. This behavior renders the Deleted Items folder significantly less useful for recovering messages that were deleted by accident, as there is little time for someone to discover the issue and recover the message prior to it being auto-cleaned.
I would like to propose that the Auto-Clean mechanism base its deletion decision on the date the message was placed in the folder being cleaned. For the Inbox, Sent Items, and Junk E-mail folders, this isn't likely to result in any signficant change in behavior, aside from the scenario where a message was moved from Inbox to another folder, then at some later date moved back to the Inbox. For the Deleted Items folder, however, this change would mean the messages would be auto-cleaned X days after they had been deleted (moved to the Deleted Items folder), rather than X days after they were originally sent/received. This would give people a reasonable amount of time to discover their error and recover the message that had been accidentally deleted.
The ultimate goal (in our case) is to be able to setup Auto-Clean on the Deleted Items folder so that users don't have to remember to periodically empty this folder to prevent their mailbox from getting full. This was never an issue with POP, but with more and more people converting to IMAP, we are continually running into issues with the Deleted Items folder filling up users' mailboxes. We have sent several communications to users that they need to perform this additional step, but it remains an on-going issue. While some mail applications have the ability to perform the purge function themselves, different applications default to different time periods, some default to not being enabled, and some don't provide that capability at all.
We would like to enable Auto-Clean on the Deleted Items folder to help minimize this issue for our users, but with the Auto-Clean behavior as it is now, it's not really an option for us, nor does it seem very intuitive.