Blackberrys don't sync email like the old Palm Pilots. The push to your BB is separate/independent of the retrieval in Outlook.
The syncing of read/unread, deleted, and sent mail between the two is a BES feature and generally not available on BIS accounts. More recently some ISPs have upgraded features so that some of the sync features of BES are available now with BIS. Apple's MobileMe is a perfect example. It was previously not even instant push, instead taking 15 minutes to be polled (the standard Blackberry cycle). Now, however, it's not only instantly pushed but you get syncing of read/unread status, deletions, and sent mail. It's still only one-way though, with changes on your BB being reflected on your Mac. Changes on your desktop don't carry through.