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

TCP/IP Electronic Mail System Overview and Concepts 1 2 3 TCP/IP Electronic Mail Communication Overview: Message Composition, Submission, Delivery, Receipt, Processing and Access

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.

TCP/IP Electronic Mail System Overview and Concepts 1 2 3 TCP/IP Electronic Mail Communication Overview: Message Composition, Submission, Delivery, Receipt, Processing and Access