Exchange Server 2003

Exchange Server 2003 (v6.5) 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.
One of the new features in Exchange Server 2003 is enhanced disaster recovery which allows administrators to bring the server online quicker. This is done by allowing the server to send and receive mail while the message stores are being recovered from backup. Some features previously available in the Microsoft Mobile Information Server 2001/2002 products have been added to the core Exchange Server product, like Outlook Mobile Access and server-side ActiveSync, while the Mobile Information Server product itself has been dropped. Better anti-virus and anti-spam protection have also been added, both by providing built-in APIs that facilitate filtering software and built-in support for the basic methods of originating IP address, SPF ("Sender ID"), and DNSBL filtering which were standard on other open source and *nix-based mail servers. Also new is the ability to drop inbound e-mail before being fully processed, thus preventing delays in the message routing system. There are also improved message and mailbox management tools, which allow administrators to execute common chores more quickly. Others, such as Instant Messaging and Exchange Conferencing Server have been extracted completely in order to form separate products. Microsoft now appears to be positioning a combination of Microsoft Office, Microsoft Office Live Communications Server, Live Meeting and SharePoint as its collaboration software of choice. Exchange Server is now to be simply e-mail and calendaring.
Source:wikipedia.org