I want to install and setup Zimbra on a Debian Squeeze installation to be our Backup MX Server, if our Exchange goes down, i want Zimbra to receive emails for our users, also would be posible to that our exchange sends every email to our zimbra server to replicate the mailboxes? Im thinking of a disaster recovery if i have a big problem with my Exchange server after doing an upgrade or because of other thing i can move the user to the zimbra server while i work on the Exchange server.

I hope somebody can help me with this, what will be the best solutions.