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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)

SMTP Replies and Reply Codes 1 TCP/IP Electronic Mail Mailbox Access Model, Method and Protocol Overview

The Simple Mail Transfer Protocol (SMTP) is responsible for most of the process of sending an e-mail message from the originator to the recipient. SMTP's job ends when the message has been successfully deposited into the recipient's mailbox on his or her local SMTP server.

In some cases this mailbox is the end of the message's travels through cyberspace. More often, however, it is only a “rest stop”—the last step of the journey is for the message to be actually accessed and read by the user to whom it was sent. This may require that it be retrieved from the mailbox and transferred to another client machine. For a variety of reasons, SMTP is not used for the process of accessing a mailbox, but rather a special set of protocols and methods designed specifically for electronic mail access and retrieval.

In this section, I describe some of the more common techniques used for TCP/IP e-mail access and retrieval. I begin with an overview of the subject that describes in general the different paradigms used for e-mail access and gives an overview of the protocols. I then have subsections that cover the two mail TCP/IP e-mail access and retrieval protocols: the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP). I also have a third, smaller subsection that discusses other methods of e-mail access.

Note that this section assumes that you already have familiarity with SMTP and the operation of the TCP/IP e-mail system as a whole.

SMTP Replies and Reply Codes 1 TCP/IP Electronic Mail Mailbox Access Model, Method and Protocol Overview