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How e-Mail Works |
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 An E-mail Message
The
first e-mail message was sent in
1971 by an engineer named Ray
Tomlinson. Prior to this, you
could only send messages to
users on a single machine.
Tomlinson's breakthrough was the
ability to send messages to
other machines on the Internet,
using the @ sign to designate
the receiving machine.
An e-mail message has always
been nothing more than a simple
text message -- a piece of text
sent to a recipient. In the
beginning and even today, e-mail
messages tend to be short pieces
of text, although the ability to
add attachments now makes many
e-mail messages quite long. Even
with attachments, however,
e-mail messages continue to be
text messages -- we'll see why
when we get to the section on
attachments |
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Bob Kahn |
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J C R Licklider |
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Ray Tomlinson |
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Tim berner lee |
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E-mail Clients
You have probably already
received several e-mail messages
today. To look at them, you use
some sort of e-mail client. Many
people use well-known
stand-alone clients like
Microsoft Outlook, Outlook
Express, Eudora or Pegasus.
People who subscribe to free
e-mail services like Hotmail or
Yahoo use an e-mail client that
appears in a Web Page. If you
are an AOL customer, you use
AOL's e-mail reader. No matter
which type of client you are
using, it generally does four
things:
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It shows you a list of all of
the messages in your mailbox
by displaying the message
headers. The header shows you
who sent the mail, the subject
of the mail and may also show
the time and date of the
message and the message size.
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It lets you select a message
header and read the body of
the e-mail message.
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It lets you create new
messages and send them. You
type in the e-mail address of
the recipient and the subject
for the message, and then type
the body of the message.
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Most e-mail clients also let
you add attachments to
messages you send and save the
attachments from messages you
receive.
Sophisticated e-mail clients may
have all sorts of bells and
whistles, but at the core, this
is all that an e-mail client
does.
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A Simple E-mail
Server
Given that you have an e-mail
client on your machine, you are
ready to send and receive
e-mail. All that you need is an
e-mail server for the client to
connect to. Let's imagine what
the simplest possible e-mail
server would look like in order
to get a basic understanding of
the process. Then we will look
at the real thing.
You know that machines on the
Internet can run software
applications that act as
servers. There are Web servers,
FTP servers, telnet servers and
e-mail servers running on
millions of machines on the
Internet right now. These
applications run all the time on
the server machine and they
listen to specific ports,
waiting for people or programs
to attach to the port . The
simplest possible e-mail server
would work something like this:
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It would have a list of e-mail
accounts, with one account for
each person who can receive
e-mail on the server. My
account name might be mbrain,
John Smith's might be jsmith,
and so on.
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It would have a text file for
each account in the list. So
the server would have a text
file in its directory named
MBRAIN.TXT, another named
JSMITH.TXT, and so on.
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If someone wanted to send me a
message, the person would
compose a text message
("Marshall, Can we have lunch
Monday? John") in an e-mail
client, and indicate that the
message should go to mbrain.
When the person presses the
Send button, the e-mail client
would connect to the e-mail
server and pass to the server
the name of the recipient (mbrain),
the name of the sender (jsmith)
and the body of the message.
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The server would format those
pieces of information and
append them to the bottom of
the MBRAIN.TXT file. The entry
in the file might look like
this:
From: jsmith To: mbrain
Marshall, Can we have lunch
Monday? John
There are several other pieces
of information that the server
might save into the file, like
the time and date of receipt and
a subject line; but overall, you
can see that this is an
extremely simple process.
As other people sent mail to
mbrain, the server would simply
append those messages to the
bottom of the file in the order
that they arrived. The text file
would accumulate a series of
five or 10 messages, and
eventually I would log in to
read them. When I wanted to look
at my e-mail, my e-mail client
would connect to the server
machine. In the simplest
possible system, it would:
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Ask the server to send a copy
of the MBRAIN.TXT file
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Ask the server to erase and
reset the MBRAIN.TXT file
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Save the MBRAIN.TXT file on my
local machine
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Parse the file into the
separate messages (using the
word "From:" as the separator)
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Show me all of the message
headers in a list
When I double-clicked on a
message header, it would find
that message in the text file
and show me its body.
You have to admit that this is a
very simple system.
Surprisingly, the real e-mail
system that you use every day is
not much more complicated than
this.
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The Real E-mail
System
For the vast majority of people
right now, the real e-mail
system consists of two different
servers running on a server
machine. One is called the SMTP
server, where SMTP stands for
Simple Mail Transfer Protocol.
The SMTP server handles outgoing
mail. The other is either a POP3
server or an IMAP server, both
of which handle incoming mail.
POP stands for Post Office
Protocol, and IMAP stands for
Internet Mail Access Protocol. A
typical e-mail server looks like
this:
The
SMTP server listens on
well-known port number 25, POP3
listens on port 110 and IMAP
uses port 143
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