Electronic Mail (E-Mail)
Another popular network application on the Internet is
electronic mail, or e-mail. E-mail allows the user to create,
send, and receive messages electronically. It's essentially the same thing as
mail that you would send through the postal service, except that the message is
sent electronically over a network, rather than physically sent using planes,
trains, and trucks. In fact, the term snail mail is often
used to refer to traditional paper mail, which takes days to deliver as opposed
to seconds.
Of course, the postal service has to overcome a lot more
difficulties than e-mail does. For example, the postal service has to physically
move tons of letters and packages, so it's reasonable that it takes a few days
to deliver letters.
E-mail works on a totally different conceptual model as
compared to the web. With web content, if someone has something to say, he
creates the content, and at some later time, anyone can look at the contents of
the web page. E-mail, however, is sent between one user and another user, or in
some cases, to multiple other users. So, the person sending the mail needs to
know the e-mail address of the person who needs to receive the e-mail. An e-mail address is a text string
that represents the address of a person for the purposes of sending and
receiving e-mail, much like a mailing address used for snail mail.
With web services, the web server needs to be connected to the
Internet before someone can browse the website using a web browser. With e-mail,
the sender and the receiver do not have to be connected to the network at the
same time. For instance, Fred can connect to the Internet from home and send an
e-mail to Barney, even if Barney isn't connected to the Internet at that time.
Later, when Barney connects to the Internet, he can check his mail and get the
e-mail that Fred sent earlier.
To allow the sender to send an e-mail, even when the recipient
is not connected to the Internet, e-mail uses the same general model that the
postal service does. If you want to send a letter via snail mail, you put it in
a nearby mailbox. The postal service moves the letter to the address of the
recipient and puts it in the recipient's mailbox. The recipient does not have to
be home when the postal carrier comes byhe just has to look in his mailbox for
today's letters when he gets home. Figure
2-8 illustrates how e-mail uses a model similar to that of the postal
service.
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