I believe they can be exposed through IMAP, although Microsoft appears to be gradually discouraging or deprecating their usage. If your version of Exchange still supports exposing them via IMAP, you could index them via the IMAP app.
Oh they might also be visible via RSS. I think there's an RSS input on Splunkbase? If not, seems like it would be a useful one, as there are many applications that provide RSS feeds of new items of interest.
