Enterprise environments have been slow to deploy Microsoft Exchange 2000 Server. The product's absolute dependency on Windows 2000, in particular Active Directory (AD), has reduced the rate at which large corporations can introduce the server. AD planning and design in a distributed network environment is complex, especially if you want to collapse a fragmented, multidomain Windows NT environment into an elegant Win2K forest. You also face the challenge of mastering AD replication, in particular deploying Global Catalog (GC) servers in a manner that can support Exchange 2000. Apart from AD, the migration from Exchange Server 5.5 involves numerous complexities, such as account migration, public-folder email-address replication, and distribution list (DL) replication in a mixed-mode organization. (For an involved discussion of these challenges, see "Managing Exchange 2000, Part 1," February 2001, "Managing Exchange 2000, Part 2," March 2001, "Managing Exchange 2000, Part 3," Spring 2001, and "Exchange 2000 Server over Active Directory," September 2000.)

Exchange Server 5.5 running Service Pack 4 (SP4) works well on Win2K and certainly will be around for a considerable time to come. Some installations might even wait to migrate to Windows XP before thinking about deploying the new generation of Exchange Server (which by that time Microsoft might have updated again). And many administrators like to wait for the first service pack before they get serious about any Microsoft product. Bitter experience has demonstrated that the wise choice is often to let other companies discover the first wave of bugs that appear during implementation, then benefit from the resulting white papers, conference presentations, books, and other deployment-review material.

Now, however, several large companies are moving ahead with Exchange 2000. Of course, Microsoft was the first to implement an organizationwide deployment, but companies such as Compaq—which at the time of this writing had migrated more than 40,000 mailboxes—are quickly catching up. Other organizations are completing Win2K deployment and preparing to start with Exchange 2000. I expect the pace to pick up throughout 2001, and I predict that Exchange 2000 will host as much as 25 percent of the total Exchange Server user base (which I estimate at more than 44 million users) by the end of the year.