Our companies' Disaster Recovery Plan calls for instant return of service after network configuration has been updated. For us the ideal backup situation would be a mirror installation of our existing Zimbra, where the existing zimbra sends a copy of every message sent and recieved by the users through the existing server off to the backup server. That way in the event of a massive failure, our users get rerouted to the hot backup host and have access to all of their email. This would also prevent deletion of messages that would later need to be retrieved for regulatory searches. This would also allow easy restore of a mailbox, I think.