Thanks for the reply and, of course, for the wonderful Aqua Mail app. I have switched to IMAP to avoid the problem I mentioned; I don't need the Exchange calendar.
I do think, however, that there will be many users who want to use Exchange but who will be plagued by the huge default address book that comes with Exchange in large organizations. In those cases auto-complete in the "Send To" field might be a nuisance and, to make matters worse, it increases the chance of sending an email to an incorrect recipient -- one with a name close in the first few letters to the the intended recipient. I would be great if the Exchange default address book search could be turned off in individual accounts.
Again, thanks for your great work.