Messaging systems depend on directories to fulfill a variety of needs. Fetching email addresses for users and knowing what server a user's mailbox is on are perhaps the most obvious, but directories also hold a lot of configuration data, such as permissions and how servers are connected together. In Exchange Server 5.5, the Directory Service (DS) stored all this data. The link between the DS and the other Exchange 5.5 components (e.g., the Store and the Message Transfer Agent—MTA) is simple: All the components reside on the same physical server. Thus, a network outage might prevent an Exchange 5.5 server from contacting other servers to transfer email or prevent users from connecting to mailboxes, but an outage won't stop the interaction between the Exchange components. Because the DS is on the Exchange server, access to the directory's data is fast and reliable.
Although having a local copy of the directory on every Exchange server delivers advantages, it implies a lot of replication traffic to keep all copies of the directory synchronized. As the number of servers grows in an organization, the replication traffic increases dramatically. Also, the Exchange 5.5 DS isn't designed for use by applications other than Exchange.
Exchange 2000 addresses these problems by supporting Active Directory (AD) and adopting a network-based directory model. That network-based model doesn't expect servers to maintain a local copy of the directory. Instead, AD is based on a set of domain controllers (DCs) and Global Catalog (GC) servers that are distributed within the network to provide directory services to applications and to the OS. DCs hold a complete copy of all the objects that belong to a domain, plus a copy of objects replicated in the forestwide configuration naming context (NC). GCs hold a complete copy of all objects in their own domain, plus partial copies of objects from all other domains within the forest.
Exchange 2000 accesses DCs and GCs for different purposes than Exchange 5.5 does. Exchange 2000 uses DCs to retrieve system configuration data, such as details about the servers within an organization. Exchange 2000 uses GCs to fetch information about user mailboxes and email addresses. The routing engine uses data from the GC to route email, and AD provides the same data, in the form of the Global Address List (GAL), to Messaging API (MAPI) clients such as Microsoft Outlook. Other clients use Lightweight Directory Access Protocol (LDAP) requests to search AD and retrieve information about recipients. Other AD consumers include the Active Directory Connector (ADC), the Recipient Update Service (RUS), and the Microsoft Management Console (MMC) Exchange System Manager (ESM) snap-in.