Microsoft recently announced a new support policy (see the first URL below) that confirms what we've all suspected for a while: The clock is ticking for Exchange Server 5.5. According to the published timeline (see the second URL below), Microsoft will no longer provide general support for Exchange 5.5 after December 31, 2003, although you can buy as many as 2 years of extended support. (This week's "Exchange and Outlook UPDATE, Outlook Edition" commentary "New Microsoft Support Policy Affects Outlook"—see the third URL below—explains and questions this support change.) The looming presence of Windows .NET Server (Win.NET Server) 2003 and the next version of Exchange (code-named Titanium) makes migration a confusing issue; planning when to migrate what can be challenging. However, Microsoft has said that you won't be able to move directly from Exchange 5.5 to Titanium through upgrading in-place servers (although you can move mailboxes from an Exchange 5.5 server to a Titanium server), so you should begin planning for a messaging-system migration sooner rather than later. How should you proceed?
First, consider which versions of Exchange run on which versions of Windows. If your servers run Windows NT 4.0, you can run Exchange 5.5 but not Exchange 2000 Server or Titanium. Exchange 2000, of course, runs on Windows 2000. At present, Win.NET Server Release Candidate 1 (RC1) doesn't support Exchange 2000 Service Pack 3 (SP3), and the Win.NET Server release to manufacturing (RTM) version probably won't either. And although speculating about products early in the development cycle is always a little risky, it's a given that Titanium will run on Win.NET Server. Microsoft probably will support Titanium on Win2K also, although running the product on Win2K will mean giving up the ability to make point-in-time snapshot copies of Exchange stores (as well as giving up a few of Titanium's other, smaller features that depend on Win.NET Server).
What does all this mean when you're considering migrating your Exchange 5.5 sites? The first and most obvious step is to move to Win2K and Active Directory (AD). You'll need to do so no matter what version of Exchange you migrate to eventually, so correctly designing and deploying AD is crucial. The first step on this path is usually to flatten your NT domain structure as much as possible before you start to migrate individual domains. Then, you can add Win2K and Exchange 2000 servers to your existing Exchange 5.5 organization. As long as you properly set up the Active Directory Connector (ADC), Exchange 2000 will run in mixed Win2K and NT 4.0 networks. You can also freely intermix Win2K member servers running Exchange 2000 with Win.NET Server domain controllers (DCs)—-a valuable feature, considering the improvements to Win.NET Server's AD and Certificate Server components. As with mixed Win2K/NT deployments, you'll be able to use Exchange 2000 in a mixed Win.NET/Win2K environment.
After you've started to sketch out a migration plan for your directory infrastructure, consider how you can get the most now out of Exchange 2000's improved scalability, Internet integration, and Outlook Web Access (OWA) support. And remember, the sooner you start planning the move, the sooner you'll be ready to deal with Microsoft's support changes.