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Email is arguably the longest lived, and remains one of the most widely used, Internet applications. As with so many other Internet-based applications, email operates on the client/server model. To send or retrieve email, the email client connects to an email server. When the client of one email server sends email to the client of another email server, the servers communicate with one another to exchange the email.

Internet-based email is always sent using the Simple Mail Transfer Protocol (SMTP). SMTP relies on TCP to open a connection to the server that is to receive the email, and then uses a set of text-based commands for communicating with the destination server and facilitating the email exchange. SMTP is used by the sending client to forward an email to the local mail server, and by the local server to forward an email to a different email server.

On the Internet, email is never delivered; it is retrieved. A client must explicitly open a connection to the server that is hosting its incoming email, and use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) to retrieve their email from the server. The primary difference between POP and IMAP is that POP simply downloads email client to server, while IMAP provides stronger support for manipulation of email on the server itself.

If the email server has been integrated with a web server, then it is also possible for someone to use a browser and the protocols of the World Wide Web (WWW) to access their email. This is a commonly supported option from Internet Service Providers (ISP) for customers who have their ISP host their email. It is also the dominant model for Application Service Providers (ASP) who provide free email services online (e.g., Google, Yahoo, MSN, etc.).


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