In order for you to do what you want you'll need one of two things:

1) An email server that stores all your messages/syncs with the desktop client
2) A third party solution/hack that acts as a conduit

Short version is that with a standard POP3 setup the sent message archive only exists on your desktop, not on the email server. Switch to IMAP4 or Exchange and the messages now live on the server.

If you setup your 750 so that it is synced with your desktop, it will automatically sync the folders between Outlook and Pocket Outlook, however that won't be a "real time" sync as it will only happen when the handheld is connected to your PC.