Ask network administrators what their most critical computer application is, and most will say "email" without a moment's hesitation. If you run a network powered by Windows 2000 or Windows Server 2003, Microsoft Exchange occupies much of your time. According to Microsoft, 110 million Exchange seats have been deployed, but 60% of you are still running Exchange 5.5. That's a problem, because the difference between version 5.5 and the more efficient Exchange 2000 and Exchange Server 2003 is profound.