You cannot remove the default
[email protected] address but you can add the appropriate address and then make it the primary address. This is done in the Exchange System Management console under the user settings.
Exchange can be configured to get the mail from your hosted service using a "Pop connector" quite easily. This would then allow the server to get the mail rather than the client, import it to exchange, and then be backed up with your server as well as allow users to sync with mobile devices, and use outlook Web Access.
Just a thought if you want another option.