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E-Mail (abbreviation for electronic mail), transmission of electronic messages between computers via a network. Millions of e-mails are exchanged every day?virtually every business relies on it and many people see it as their preferred method of communication.For all its ubiquity and importance e-mail is very simple and has humble roots. An e-mail message is nothing more than a piece of text and the sending of a Mail message requires no more than the simple attachment of this text to a special file, known as a mailbox.The first e-mail message was sent in 1971 by an engineer named Ray Tomlinson, one of the pioneers of the Internet. He showed how a messaging facility that could be used by several users on a single computer could be extended so that it worked between a numbers of computers. Tomlinson decided that the @ sign should be used to designate the receiving machine, and so e-mail as we know it was born.Practical e-mail systems have only a few, simple components. At the user end is a piece of software known as an e-mail client Microsoft Outlook, Eudora, Pegasus, and Web-based clients such as America Online Inc. (AOL's) e-mail reader are familiar examples. The client allows the user to create mail messages, to view the contents of the mailbox, and to read incoming mail.At the other end from the client is the e-mail server. This is a computer, typically one provided by an Internet Service Provider that is dialed up when messages are sent and received. The server has a list of e-mail accounts, each of which has a text file where all of the messages for that account are stored.To send an e-mail, a message is created using the client and is sent to the server. The server forwards the message to the computer that hosts the mailbox of the intended recipient. To receive e-mail, the user simply logs on to the e-mail server, which presents a set of message headers to the client. These headers provide information as to who sent the message and when it was sent.There are two protocols that govern how real e-mail systems work. The first is SMTP (Simple Mail Transfer Protocol), which handles outgoing mail. The other is POP3 (Post Office Protocol), which handles incoming mail, simply appending incoming messages to a user?s file. SMTP ?listens? for any attached clients who want to send messages on the server?s well-known port number 25, while POP3 listens for messages addressed to one of its clients on port 110.E-mail messages get from one server to another in exactly the same way that any other information traverses the Internet. A program called sendmail is used to queue outgoing messages so if a network link is broken, the message will be re-sent until it gets through.Despite the fact that e-mail only works for text, a variety of file types (for instance, images, sounds, spreadsheets, and so on) can be attached. A program called ?uuencode? turns all attachments into text so that they can be transmitted across a network. When the message (which could be some words plus an attachment rendered into text by uuencode) is received, the client invokes ?uudecode? to restore the original.

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