Is there a command to do this? I can get messages since a certain date, or RECENT or UNSEEN, but I can't find any way to just get the newest message and only the newest message.
When you connect, the server gives you an EXISTS, telling you how many messages you have. Take that number (say, 62), and fetch that message: tag FETCH 62 (whatever). That's the most recent message that's been added to the mailbox.