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 solutions. 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 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. Microsoft Exchange Server is the industry standard for corporate email.

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Exchange Server 2003 (v6.5, code name Titanium) debuted on September 28, 2003. Exchange Server 2003 (currently at Service Pack 2) can be run on Windows 2000 Server (only if Service Pack 4 is first installed) and 32-bit Windows Server 2003, although some new features only work with the latter. Like Windows Server 2003, Exchange Server 2003 has many compatibility modes to allow users to slowly migrate to the new system. This is useful in large companies with distributed Exchange Server environments who cannot afford the downtime and expense that comes with a complete migration.

The June 2, 2003, release of Exchange Server 2003 made the migration from pre-2000 versions of Exchange significantly easier (although still involved the same basic steps), and many users of Exchange Server 5.5 waited for the release of Exchange Server 2003 to upgrade. The upgrade process also required upgrading a company's servers to Windows 2000.

Exchange Server 2007 was released on November 30, 2006, to business customers as part of Microsoft's roll-out wave of new products. It includes new clustering options, 64-bit support for greater scalability, voice mail integration, better search and support for Web services, better filtering options, and a new Outlook Web Access interface. Exchange 2007 also dropped support for Exchange 5.50 migrations, routing groups, admin groups, Outlook Mobile Access, X.400, and some API interfaces, amongst other features.

Exchange Server 2007 (v8, code name E12, or with SP1 v8.1) runs only on 64-bit x86-64 versions of Windows Server. This requirement applies to supported production environments only; a 32-bit trial version is available for download and testing. Hence, companies currently running Exchange Server on 32-bit hardware will be required to replace or migrate hardware if they wish to upgrade to the new version. Companies that are currently running Exchange Server on 64-bit capable hardware are still required to migrate from their existing Exchange 2000/2003 servers to a new 2007 server since in-place upgrades are not supported in 2007.