Build it like you would build a multitier web site (Score 1)

You should look at the following: - A good L4 Load balancer (Foundry, F5, Cisco, etc...) - A good platform for inbound and outbound email filtering and relay to front exchange; appliances exist that do this well (Ironport, Barracuda) and you can scale these via the aformentioned Load Balancer - Build a good LDAP Directory Infrastructure; Sun makes a very good one - it will scale into 10's of millions of users - Run your Exchange servers in VMware (the Data Center version) where you can quickly recover and backup your servers since the image is just a file - Implement Exchange on a good SAN such as EMC or Hitachi