BackupAgent will release an update (3.13.2.1) for the Windows backup client next month. In this update we will support the possibility to connect to an Exchange Server running in the domain from a different machine. This feature has been designed to support mailbox level backup of Exchange 2007.
Details: http://www.backupagent.com/en/blog/archive/2009/09/28/remote_mailbox_backup_of_exchange_2007