The outlook/microsoft exchange does not support the full IMAP standard and changes the UID of the email every time you click on it, reply to it, open it, or forward it. The blackberry server only checks for the UID, and if it's not the same as the one on your phone, it'll download it again.
I really wish either the Blackberry server would check for more than the UID (Gmail does it), but so far it seems neither company will budge. Microsoft wants to do it their way (go figure) and RIM wants to adhere to the standard (makes sense).
Really, I think RIM should just back down from their stronghold and check for more than UID to determine if an email is new or not.
