We are in final stages of evaluating Zimbra NE as a replacement for our current mail system, with Exchange as the other candidate.
The ability to provide at least a cold standby (at a remote location) is a major requirement, and I believe I've worked out procedures which will allow me perform frequent scheduled data synchronization and then bring up the standby should it be needed. Some of this is discussed in these threads:
Mac OS X Leopard Beta: compatibility with Tiger
Of backups and restoration
However I've also been asked to provide a backup mx in the remote location. I suppose the simplest approach would be to just run something on another (third) machine, but I'd like to be able to offer a more elegant solution that minimizes hardware and physical space requirements. In short, ideally, the standby machine would be able to operate as a secondary mx, and if I need to make the sync'ed data "live", I would be able to integrate any mail that had arrived in the meantime.
My first thought on this would be to have some steps where the MTA being used as a secondary mx would be configured to stop listening on standard ports before Zimbra is made active. Then I could tell it to go ahead and deliver its mail to Zimbra on the same machine. (I'd possibly multihome and have the two MTAs on different IP addresses if necessary.)
I'm looking for advice on whether this sounds feasible and any suggestions on proceeding. This would likely be done under Mac OS X 10.4 or 10.5, so any of the common MTAs available for that platform could be used, or even Communigate, which is our current mail server and could be relegated to that role.
I also wonder if I'm making this more complicated than it needs to be. E.g. is there a way to simply submit mail directly by copying files? Or perhaps a way to run Zimbra (FOSS) as a backup mx and then integrate the outgoing mail queue when I bring up the NE?
Will the next major release of Zimbra offer anything to help with this--hopefully without requiring purchase of multiple NE licenses?