Server applications can be developed either as extensions to the Microsoft Exchange Server Administrator program or as stand-alone applications. The Administrator program performs basic operations such as creating mailboxes, managing basic Microsoft Exchange Server components, and managing directory objects. You can extend the Administrator program by adding custom property pages, creating new data fields for a directory object, and adding application-specific extension data.
Stand-alone server applications for Microsoft Exchange Server interact with user services such as the information store and the directory. Server application components can reside on a server running Microsoft Exchange Server, or on any computer running Windows NT Server, even if that system is not running Microsoft Exchange Server. Most stand-alone server applications are either mailbox agents or gateways. Mailbox agents are server applications that are accessed through a mailbox on a server. Gateways are interfaces with other mail systems.