I think they made something really revolutionary at the IE3 time. Their News and Mail app was an Explorer extension that placed an e-mail reader as the presentation of a folder full of folders of mailboxes and messages. You wouldn't see the extension, as the apps launched as applications, but that's what the implementation looked like from what I investigated back then.

Unfortunately, the idea was seemingly abandoned almost immediately. I would love to have such views on top of a user-space file system keeping messages, address books, and calendars in sync.