The native Internet mail systems are going to push out proprietary systems, and the main reason for that is security. Currently, you've got a lot of proprietary mail systems that use gateways to get the mail from the proprietary mail system to the Internet and then over to a different organization. A gateway, by definition, changes a message. Or if the message is encrypted, a gateway can't even see the message content, much less change it. In general, I find that hard to predict.
People are very ingenious in finding new ways to apply technology.
But one of the interesting areas is shared folders. With IMAP, users can change the access control list on one of their mail folders, so that others can access them. They can set the folder, for instance, to read-only, insert, write, that sort of thing. Managers can give their assistants read-only access to their mail, so the assistant can screen it and copy the mail into folders where the manager can then read the filtered mail. That's a big change.