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I'm not sure I agree with your description of the server being a PST but it's not worth redescribing here. What I would note though is that the OST isn't all that different from a PST and when getting large could cause the same issue. Multiple PSTs - yes but as mentioned, only if older ones are closed. I think it's unrealistic to expect end users to remain concious of these things though.

Backing up PSTs is an interesting concept but I can't see it being manageable in a larger environment.

Why no mention of Exchange 2010 and its archive DBs? There are also 3rd party archive DB solutions.