I understand this is experimental but we are attempting to set this up so we have a warm stand by mail server ready for emergency reasons. This solution seems perfect for us.

Server Live sync - Zimbra :: Wiki

Have followed all of the steps but run into a major problem. Both machines use the same IP(only one zimbra is up at anytime.) Problem is when We do the rsync from the live server to the mirrored server from /opt/zimbra to /opt/zimbra everything seems to copy fine but when you go to bring up the mirrored server it says it is starting from the live server hostname and just fails. Wondering if there is anyway around this. Thanks.