[CentOS] clustered mail server?

Hi all

I'm looking for a working HOWTO / Tutorial / sample setup of setting up a clustered email server. I know I can setup a CentOS cluster with Heartbeat & drbd, but I'm not 100% convinced it will work for the email as well.

Ideally, I'd like to use an open source groupware server, like say open exchange, php groupware, Scalix (free / community version), etc. So, if anyone can please point me in the right direction?

I've already setup a heartbeat cluster, and use rsync to sync files for Samba, Apache & NFS, and also setup a Master-Master MySQL replication, but don't yet know how to setup the email server,