I have not tried it but If you have a backend storage solution for ISCSI or NFS I would give proxmox VM clustering a try. If you are a Redhat fan you might want to look into ovirt.org which is a RedHat sponsored VM server solution that runs on Fedora 18 and supports clustering. Might also be worth mentioning that both nginx and pound can be used to load balance mail server traffic.
http://pve.proxmox.com/wiki/High_Availability_Cluster
http://www.ovirt.org/OVirt_3.0_Feature_Guide
http://www.apsis.ch/pound
http://nginx.org/en/
I hope this helps
-Tim