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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 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

The need to communicate is as old as humanity itself. Thousands of years ago, communication was, of necessity, almost exclusively local. Messages were primarily oral, and even when in writing, rarely delivered a great distance. Most people never travelled far from their homes, and rarely communicated with those distant from themselves. But even in ancient times, messengers were used by leaders to send short pieces of critical information from place to place. It was slow and unreliable, but some messages were important enough that an effort to communicate often had to be made in spite of the difficulties.

Advances in transportation led to advances in communication capability, eventually resulting in the creation of physical mail systems. Today, these systems have evolved to the point where anyone in the developed world can send a letter to just about anyone else. Reliability has vastly improved, despite all the jokes people make about the postal service. J Speed is also much better than it was in the olden times, with messages now taking days to reach their destination instead of weeks or months.

Waiting even days for a message to get from one place to another is pretty slow by the standards of our modern world. For this reason, one of the most natural applications of networks was to use them as a replacement for the physical transportation of messages from one place to another. Transforming mail from a physical process to an electronic one yields enormous benefits, chief among them greatly increased communication speed, the ability to instantly send one message to multiple recipients, and the ability to get nearly instantaneous feedback upon receipt of a message.

The idea behind electronic mail (e-mail or email) is not only as old as computer networks, it actually predates internetworking. The first electronic mail systems were implemented on traditional mainframe computers. These are single large computers accessed by many users simultaneously through connected terminals. An e-mail system on a mainframe consisted of a set of software running on the mainframe that implemented the entire electronic mail system. Each user simply had a mailbox that resided on this machine, and mail was “delivered” by moving messages from one mailbox to the next. Users sent and received mail through a user interface program.

Such an early electronic mail system was useful for local communication, but not for sending messages to a person in another organization. Mainframe e-mail is somewhat analogous to “local mail” being sent by one resident of a town to another. There is no way to send mail to a person in a distant town without more infrastructure for delivery.

The power of internetworking is what really enables electronic mail to become a universal method of communication. Internetworks link together systems the way the postal service's fleet of airplanes and vehicles link together post offices. Mail is sent from user to user over the underlying technology of the internetwork. TCP/IP is of course the most commonly used internetworking protocol suite, and the modern Internet uses TCP/IP to tie together systems across the globe. It is thus “the vehicle” for sending electronic mail.

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