Oh, here's another good one. Despite the fact that you can assign permissions to Exchange mailboxes such that you can let other people access your email through Outlook, Exchange doesn't pass those permissions through the IMAP server, despite IMAP having a very similar permissions mechanism. Not that I even want to be able to modify the permissions through IMAP; I'd just like to be able to access the mailboxes I do have permissions to, and Exchange doesn't even show me that they exist via IMAP.