Microsoft Exchange Server is the server side of a client–server, collaborative application product developed by Microsoft. It is part of the Microsoft Servers line of server products and is used by enterprises using Microsoft infrastructure products. Exchange's major features consist of electronic mail, calendaring, contacts and tasks; support for mobile and web-based access to information; and support for data storage.
Planning the migration from Microsoft's internal "legacy XENIX-based messaging system" to the Exchange Server environment began in April 1993, and by January 1995 some 500 users were running on Exchange Server Beta 1. By April 1996 32,000 users were migrated to that environment.
Exchange 2000 Server (v6.0, code name Platinum), released on November 29, 2000, overcame many of the limitations of its predecessors. For example, it raised the maximum sizes of databases and increased the number of servers in a cluster from two to four. However, many customers were deterred from upgrading by the requirement for a full Microsoft Active Directory infrastructure to be in place, as unlike Exchange Server 5.5, Exchange 2000 Server had no built-in Directory Service, and had a dependency upon Active Directory. The migration process from Exchange Server 5.5 did not have any in-place upgrade path, and necessitated having the two systems online at the same time, with user-to-mailbox mapping and a temporary translation process between the two directories. Exchange 2000 Server also added support for instant messaging, capability was later spun off to Microsoft Office Live Communications Server.