So you want to eliminate a single point of failure - where do you start?

Let's assume you have anything upwards of 100 users. You would like mail flow, OWA, Active Sync and Outlook to carry on working - what's the minimum hardware requirement ?

4 Servers. No matter if you have 100 or 2000 users, the minimum stays roughly the same. The servers specs would change however. With this in mind and our 100 users, lets explain why 4 servers.

With the advent of Exchange 2007 SP1 comes CCR - Cluster Continuous Replication - as far as I'm concerned - the best thing since sliced bread with butter! Minimum spec for better than sliced bread - three machines.

Two mailbox roles, and one other - preferably a hub transport server. However we did say we needed redundancy, so throw in another machine in case the first HT (Hub Transport) dies, and we have 4 servers.

What about CAS ? CAS can happily co-exist on a HT AND both HT and CAS support NLB. HT doesn't need NLB for mail transport, however if you have internal customers submitting mail via SMTP then NLB is rather nice.

So in summary - minimum requirement for any kind of redundancy -  two times mailbox plus two times HT/CAS combos =  4 servers.