I do not know of a way to accomplish this. Most “redundancy” scenarios are intended to be for the server itself and all of the Information store.
The best method I can think of would be good backups of the server and exchange databases with a backup client. You could alternately have any e-mails that go to a user also go to another mailbox in addition to the user themselves via teh delivery options– but this would be a record of all messages and include ones that were deleted.
*** Updated by Wrobinson on 4/17/08 ***
No this is not possible. You can install an Exchange Server 2003 server cluster; however, there are specific hardware, networking and shared storage requirements. Since it is only for VIPs, the number of users will probably be relatively small, so you may be able to use direct attached storage which will help reduce the cost versus a centralized storage area network.
The XOsoft and Double-Take solutions use database replication from a source server to a target server; fail-over in the case of an outage can be an automated or manual process based on the requirements of the environment and personal preference.