Are you an Exchange shared hosting provider? Or just curious about what's included with HMC 4.0? If you can answer yes to at least one of these questions, then I recommended you give the new HMC 4.0 - Microsoft Exchange Server 2007 Service Pack 1 White Paper.
This white paper introduces the Microsoft Exchange Server 2007 Service Pack 1 (SP1) new features related to the Microsoft Solution for Hosted Messaging and Collaboration version 4.0. In particular, it addresses key features in anywhere access, built-in protection, and operation efficiency.