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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

POP3 Update State: Mailbox Update and Session Termination Process and Commands 1 IMAP Overview, History, Versions and Standards

The offline mailbox access model provides the basic mail access functions that most users need. Using the popular Post Office Protocol (POP3), a user can access his or her mailbox and retrieve messages so they can be read on a local machine. This model has the advantage of simplicity, but does not provide many features that are increasingly in demand today, such as keeping track of the status of messages and allowing access from many client devices simultaneously. To provide better control over how mail is accessed and managed, we must use either the online or disconnected access models. The Internet Message Access Protocol (IMAP) was created to allow these access models to be used, providing rich functionality and flexibility for the TCP/IP e-mail user.

In this section I describe the operation of the Internet Message Access Protocol. I begin as I usually do, with a general look at the protocol, its history, versions and standards. This includes a discussion of its benefits compared to the simpler POP3. I describe the operation of the current version of the standard, IMAP4, focusing on how client and server devices communicate and showing how the IMAP session moves through a series of four states. I describe how IMAP commands, results and responses are used and discuss the commands that can be used at any time during an IMAP session. I then describe the processes that are performed and commands used in each of the three main IMAP states.

POP3 Update State: Mailbox Update and Session Termination Process and Commands 1 IMAP Overview, History, Versions and Standards