Overview of the Exchange 2000 Architecture

The architectural changes made in the transition from Exchange 5.5 to Exchange 2000 are massive. (See the illustration “Architectural Changes in Exchange 2000“.) Microsoft has rebuilt the basic architecture, partitioning four core services so that each can run on multiple machines.

At the heart of the previous release, Exchange 5.5, are two databases: the Exchange Directory and the Information Store (IS). The first stores configuration information required by the messaging system, such as information about users, e-mail addresses, and distribution lists. (For security reasons, some user information, such as usernames and passwords, are stored separately on a Windows NT Domain Controller.) The second database, the IS, holds communications protocol information and user-created data, such as mailbox messages, calendar event information, and files located in Public Folders.

Another Exchange 5.5 component, the Message Transport Agent (MTA), is responsible for transporting and routing messages between sending and receiving servers. Finally, the System Attendant handles background processes such as security and building routing tables for the MTA. These four services formed the core of the Exchange architecture, from its initial release (at version 4.0) to the current production release, 5.5.