I have 2 mailservers running ZIMBRA (primary - site1) and (secondary - site2) in two separated locations (sites).
When my primary mailserver in AVAILABLE the seconray are just syncing (rsync) with his zimbra services down. Just when the primary goes down is that my secondary mailserver starts the Zimbra services and became available to receiving email messages.
How my internal users (webmail and clients like thunderbird/outlook) in the primary location (site1) will send the messages to secondary server if my Primary MailServer at SITE1 goes down? An email coming from outside it's OK, because we have 2 MX entries (primary and secondary mailservers), but the internal users in the same network dont send the email messages to MX server, they deliver the message directly to mailserver.
This is how my setup looks like: http://postimg.org/image/l2l7aziex/