TABLE OF CONTENTS Exchange 2003 (and Exchange 2000 before it) is radically different from Exchange 5.5. Architecturally, it's a world away from the self-contained messaging system that Exchange 5.5 was, but compared to Exchange 2000 it has much in common and differs really only by refining functionality and making improvements here and there. Those components that were built into Exchange 5.5 had been separated out from Exchange and, in most cases, integrated into the underlying operating system. For Exchange 2000 that operating system was Windows 2000. For Exchange 2003 you have the choice of two operating systems on which to run the product: Exchange 2003 can run on both Windows 2000 and Windows 2003.
Exchange 2003 positively requires at least Windows 2000 Service Pack 3 in order to operate. The Exchange 2003 executables don't run under any earlier versions of Windows, and in addition, to have an Exchange 2003 mailbox, you must have at least a Windows 2000 account. Why is there such a dependency? Well, there are a variety of reasons, but one of the most important reasons is the Active Directory (AD).
Exchange 5.5 had a reliable and distributable directory service built-in. Now there is no directory service built into Exchange: Exchange 2003 relies completely on the AD to store information about mailboxes, custom recipients, and distribution lists, although these Exchange 5.5 primitives are represented differently in Exchange 2003 and the AD. Additionally, all of the configuration information about Exchange servers that was previously held in the Configuration container...
