Mailbox-distributed is a project mainly written in Java, it's free.
Implementation of utils to use james mailbox implementations in a cluster
This project contains helper classes which make it easy to support clustering for Apache James Mailbox (http://james.apache.org/mailbox) implementations.
This is done by using hazelcast for all the distribution operations.
By using this classes it should be really straight forward to build an Apache James Server cluster. (http://james.apache.org/server)