The Microsoft Exchange Server system from a protocols perspective, where the server provides protocols for clients, is illustrated in the following figure. The clients that interoperate with the server perform messaging tasks, and ancillary entities provide essential supporting services.
Each protocol exposes a set of functionality that pertains to specific classes of operations. For example, the Simple Mail Transfer Protocol (SMTP), the Post Office Protocol - Version 3 (POP3), and the Internet Message Access Protocol - Version 4 (IMAP4) constitute a set of Internet Standard protocols that simple e-mail clients use to send, retrieve, and manage e-mail messages; Exchange Web Services offers a standardized interface for middle-tier applications to build value-added services; the Web Distributed Authoring and Versioning Protocol (WebDAV) provides a set of interfaces that caters to distributed authoring; and the remote operations (ROPs) along with either the remote procedure call (RPC) interface or the MAPI extensions for HTTP provide all of the above as well as direct access to storage and retrieval services.
In the simplest sense, the Exchange server operates under the common client-server architecture, where a messaging client connects to an Exchange server by using one or more of the available protocols. The client performs tasks by issuing a series of requests to the server and processing server responses. Behind the simplicity of the client-server architecture lies functionality from basic storage to accessing, updating, and synchronizing address books, appointments, and shared folders.