Of all things, looks like Outlook Express might accomplish some of what I need. That is, it does seem to open up folders with new messages. And the "download all messages" setting gives me a reasonable offline view. Unfortunately, there are quite a lot of holes, as well:

Major Issues

  • Inability to create folders-within-folders. OE just doesn't get it. It will read subfolders created by some other means, so I can always work around this. Annoying to lose the ability to create subfolders on the fly, though.
  • Slow. Very. Notorious for opening and closing connections in a very un-IMAP way.

Minor Issues

  • No new-mail notification on items in subfolders. Or at least, no reliable notification. No worse than the rest, here.
  • No disconnected-mode support. Not a unique problem, by any means.

On balance, not clear that just opening folders outweighs the inconveniences. It's just a bit better than Outlook, and if things are that close, why not stick to the same program that houses my address book and calendar?