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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 System Overview and Concepts

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TCP/IP Electronic Mail System Overview and Concepts
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TCP/IP Electronic Mail Communication Overview: Message Composition, Submission, Delivery, Receipt, Processing and Access
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TCP/IP Electronic Mail System Overview and History
(Page 3 of 3)

Overview of the TCP/IP Electronic Mail System

One of the most important general concepts in the modern electronic mail system is that a distinction is made between protocols that deliver electronic mail between SMTP hosts on the internetwork, and those that let users access received mail on their local hosts. To continue our analogy, different protocols are used for sending mail between post offices, and for “home delivery”. As we'll see, this was done intentionally, to make it possible to send mail to users even if they are not connected to the Internet at the time mail was sent. This decoupling is critical as it enables delayed communication, where mail can be sent when the sender wants to transmit it, and received when the recipient wants to read it.

Over the years, the basic components defined in the early 1980s have not changed substantially, but how they are used has evolved and been improved. Early electronic mail delivery involved the use of route specifications by one SMTP host to dictate how mail was to be delivered through intermediate systems; today, the Domain Name System makes much of that obsolete, facilitating nearly immediate direct mail delivery in most cases. Early electronic mail supported only simple text, where we can now send graphical images, programs and other files in e-mail. Modern high-speed Internet connections and updated access protocols allow modern e-mail to be the realization of the ultimate goal of nearly instantaneous communication even across continents.

Key Concept: One of the most important TCP/IP applications is the internetworking equivalent of the real-world postal delivery system, commonly called electronic mail or e-mail. The history of e-mail goes back to the very earliest days of TCP/IP’s development; today it is used by millions of people every day to send both simple and complex messages around the world. TCP/IP e-mail is not a single application, but rather a complete system that includes several protocols, software elements and components.



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TCP/IP Electronic Mail Communication Overview: Message Composition, Submission, Delivery, Receipt, Processing and Access
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