We face much the same problem. In order to archive mail Outlook needs to be configured to have an archive PST file somewhere. Where that is can vary, but I believe the default is somewhere in the user's
My Documents folder. This can be changed. During the archive process mailbox objects (messages, appointments, todo lists, whatnot) are compared versus the AutoArchive settings for that mailbox or specific folders. Stuff that's older than a certain point is either copied to the PST file, or deleted. If you have it enabled, these messages will remain in 'Recover deleted items' space for some time.
Cached mode is much more efficient than direct connect mode. Some users who have been in direct connect mode don't like moving to cached mode since email turn-around time (the time it takes to hit send to a distribution list the user is a member of and when it actually shows up in the mailbox) is noticeably slower in cached mode. Potentially a lot slower. However, if they've never been in direct-connect mode, there is no reason to make them.
The one thing about cached mode is that it also caches the Global Address List. So if you're making changes to GAL data, it won't show up on cached-mode user's GAL lists until some time has passed. Again, if this doesn't bother you then don't worry about it.
It sounds like you need to review the procedures for configuring Auto Archive settings on your desktops. That should tell you how things are being handled (is auto-archive deleting things, or copying to a pst file somewhere?) as well as suggest remedies.
Handling archive files without a true Email Archive platform is tricky. About the only methods available to Exchange/Outlook environments are:
- Leave everything in the mailstores
- Deal with PST archive files on an individual basis
Most organizations opt for the second bullet point.