Microsoft's IMAP implementation turned out rather poorly.
Not only it does not have IMAP IDLE (the industry standard for push mail, used by Gmail, AOL, Fastmail, even Exhange), but it lacks some other, more basic, IMAP features (e.g. UIDPLUS for efficiently moving messages between folders).
It can also error out when mail apps try to connect and authenticate, esp. when there are many messages in your Inbox (IIRC, "many" here means "more than one or two hundred").
So, all in all, a big disappointment so far, and is the reason why the preset in Aqua is still POP3.
