Basically, the Blackberry BIS contacts your pop or imap server and delivers emails to the phone. WM (and many other devices) contain built-in pop or imap clients. Those devices access the email server directly. The polling of the server to find out if there is new mail is just one of the inefficiencies that doesn't occur. Yes, WM does have a similar "push" technology, but you've got to have Exchange as the backend.