Well if the user has been using the Outlook or Outlook Express as POP3 email, then the user has their pst file(s) on their machine.
If they are to berestricted to Exchange emails, then from Control Panel Select mail (while Outlook is closed) and modify the profile Email Account to use Exchange. If you are using domain User accounts, then it will automatically be picked. Make sure that the original data pst file is selected. Edit email account and modify the Exchange account so that the default data file is the pst file rather than the exchange Mailbox.
When the user restarts Outlook, two folders are noticed, that of the exchange Mailbox and the pst folder. At Send/Recive, the email will be synchrinised.
The above is the optimum, as the users connecting remotely will see only the new emails that are still in the exchange mailbox