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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
                     9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods

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TCP/IP Electronic Mail Mailbox Access Model, Method and Protocol Overview
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POP Overview, History, Versions and Standards
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TCP/IP Post Office Protocol (POP/POP3)

The overall communication model used for TCP/IP e-mail gives many options to an e-mail user for accessing his or her electronic mailbox. The most popular access method today is the simple offline access model, where a client device accesses a server, retrieves mail and deletes it from the server. The Post Office Protocol (POP) was designed for quick, simple and efficient mail access; it is used by millions of people to access billions of e-mail messages every day.

In this section I describe the operation of the Post Office Protocol, focusing on version 3 (POP3). I begin by looking at the protocol in general terms, discussing its history, the various versions of the protocol and the standards that define them. I describe POP3's general operation and the communication between a client and server, concentrating on the three main states through which the session transitions. I then describe each of these states in sequence: the Authorization state, Transaction state and Update state. For each, I explain the communication processes that take place and the commands and replies that implement them.

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