My advice would be to get a good UPS for the Zimbra server and use a domain hosting service that will collect your mail if your server is down. I use
EasyDNS (there are others if you do a search on google/yahoo) and they hold mail for up-to five days and when the server is back on-line it delivers the mail. The basic service is not free but reasonably priced and you get professional mail server that you don't have to worry about, you don't really want a backup mail serveras they're a good target for spammers.