Setting up LDAP loadbalancing for Zarafa
From Zarafa wiki
When integrating Zarafa to an Active Directory or OpenLDAP server, the Zarafa server can only be connected a single LDAP server. In case there is a failure with this LDAP node, the Zarafa server will be unreachable.
To solve this single point of failure the a LDAP loadbalancer can be used with connects to two or more LDAP servers. For LDAP loadbalancing there different commercial and opensource solutions available.
A simple opensource loadbalancing tool which can used for this is Pen
This article describes how Pen can be configured for LDAP loadbalancing and High Availability for Zarafa.