Exchange 2003 uses Active Directory to store and share information with Windows. Therefore, all the directory information that you create and maintain in Windows, such as organizational unit structure and groups, can also be used from Exchange.
The Active Directory schema can be extended to include custom attributes and object types to centralize and minimize data administration, and also to make data available to applications that can access Active Directory information. In fact, when you install your first Exchange server, Exchange 2003 extends the Active Directory schema to include Exchange-specific information. Extending the schema affects the whole forest and, depending on the size of Active Directory, may take a significant amount of time to complete.
Because Active Directory serves as a single-source directory for all the objects in your organization, Exchange uses this information to reduce administrative overhead. With Active Directory, you can store and organize information about users, such as names, e-mail addresses, and phone numbers. This information is stored as attributes of the user object. Exchange and other applications can use this information.