If I read a message in my Exchange mail account through a client other than Evolution (such as via a cell phone or through OWA), the message is marked as read in Evolution, but the count of unread messages listed next to the folder (inbox, in this case) remains unchanged.  The only way to get the count to reset is to restart Evolution (refreshing the folder does not help, nor does disconnecting and reconnecting Evolution).  I don't know if it's significant or not,  but the "unread mail" icon works correctly, in that it doesn't show any unread messages, even though the unread count for the inbox is non-zero.

I have seen many bugs about this behavior for IMAP (where I guess it is being intentionally ignored?), but I didn't find this issue reported with MAPI.