Every day, the citizens of the Internet send each other
billions of e-mail messages. If you are online a lot, you
yourself may send a dozen or more e-mails each day without
even thinking about it. Obviously, e-mail has become an
extremely popular communication tool.
Have you ever wondered how e-mail gets from your desktop to
a friend halfway around the world? What is a POP3 server, and
how does it hold your mail? The answers may surprise you,
because it turns out that e-mail is an incredibly simple
system at its core! In this edition of HowStuffWorks,
we'll take an in-depth look at e-mail and how it works!
An E-mail Message
According to this
(extremely interesting) article, 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
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
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.
- 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
- It lets you select a message header and read the body of
the e-mail message.
- 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.
- Most e-mail clients also let you add attachments to
messages you send and save the attachments from messages you
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
If you have read How Web
Servers and the Internet Work, then 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 (see How Web
Servers and the Internet Work for details). The simplest
possible e-mail server would work something like this:
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.
- 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.
- 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.
- 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.
- 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:
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:
When I double-clicked on a message header, it would
find that message in the text file and show me its body.
- Ask the server to send a copy of the MBRAIN.TXT file
- Ask the server to erase and reset the MBRAIN.TXT file
- Save the MBRAIN.TXT file on my local machine
- Parse the file into the separate messages (using the
word "From:" as the separator)
- Show me all of the message headers in a list
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!
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 a POP3 Server, where POP stands for Post
Office Protocol. The POP3 server handles incoming mail. A
typical e-mail server looks like this:
The SMTP server listens on well-known port number 25, while
POP3 listens on port 110 (see How Web
Servers and the Internet Work for details on ports).
The SMTP Server
Whenever you send a piece of
e-mail, your e-mail client interacts with the SMTP server to
handle the sending. The SMTP server on your host may have
conversations with other SMTP servers to actually deliver the
Let's assume that I want to send a piece of e-mail. My
e-mail ID is brain, and I have my account on
howstuffworks.com. I want to send e-mail to
email@example.com. I am using a stand-alone e-mail
client like Outlook Express.
When I set up my account at howstuffworks, I told Outlook
Express the name of the mail server --
mail.howstuffworks.com. When I compose a message and
press the Send button, here is what happens:
If, for some reason, the SMTP
server at How Stuff Works cannot connect with the SMTP server
at Mindspring, then the message goes into a queue. The SMTP
server on most machines uses a program called sendmail
to do the actual sending, so this queue is called the
sendmail queue. Sendmail will periodically try to
resend the messages in its queue. For example, it might retry
every 15 minutes. After four hours, it will usually send you a
piece of mail that tells you there is some sort of problem.
After five days, most sendmail configurations give up and
return the mail to you undelivered.
- Outlook Express connects to the SMTP server at
mail.howstuffworks.com using port 25.
- Outlook Express has a conversation with the SMTP server,
telling the SMTP server the address of the sender and the
address of the recipient, as well as the body of the
- The SMTP server takes the "to" address
(firstname.lastname@example.org) and breaks it into two parts:
If the "to"
address had been another user at howstuffworks.com, the SMTP
server would simply hand the message to the POP3 server for
howstuffworks.com (using a little program called the
delivery agent). Since the recipient is at another
domain, SMTP needs to communicate with that domain.
- The recipient name (jsmith)
- The domain name (mindspring.com)
- The SMTP server has a conversation with a Domain Name
Server, or DNS (see How Web
Servers and the Internet Work for details). It says,
"Can you give me the IP address of the SMTP server for
mindspring.com?" The DNS replies with the one or more IP
addresses for the SMTP server(s) that Mindspring operates.
- The SMTP server at howstuffworks.com connects with the
SMTP server at Mindspring using port 25. It has the same
simple text conversation that my e-mail client had with the
SMTP server for How Stuff Works, and gives the message to
the Mindspring server. The Mindspring server recognizes that
the domain name for jsmith is at Mindspring, so it hands the
message to Mindspring's POP3 server, which puts the message
in jsmith's mailbox.
The actual conversation that an e-mail client has with an
SMTP server is incredibly simple and human readable. It is
specified in public documents called Requests For
Comments (RFC), and a typical conversation looks something
What the e-mail client says is in red, and
what the SMTP server replies is in green. The e-mail client
introduces itself, indicates the "from" and "to" addresses,
delivers the body of the message and then quits. You can, in
fact, telnet to a mail server machine at port 25 and
have one of these dialogs yourself -- this is how people
250 mx1.mindspring.com Hello abc.sample.com
[184.108.40.206], pleased to meet you
mail from: email@example.com
250 2.1.0 firstname.lastname@example.org... Sender ok
rcpt to: email@example.com
250 2.1.5 jsmith... Recipient ok
354 Enter mail, end with "." on a line by itself
John, I am testing...
250 2.0.0 e1NMajH24604 Message accepted
221 2.0.0 mx1.mindspring.com closing connection
Connection closed by foreign host.
You can see that the SMTP server understands very simple
text commands like HELO, MAIL, RCPT and DATA. The most common
- HELO - introduce yourself
- EHLO - introduce yourself and request extended
- MAIL FROM: - specify the sender
- RCPT TO: - specify the recipient
- DATA - specify the body of the message (To:,
From: and Subject: should be the first three lines.)
- RSET - reset
- QUIT - quit the session
- HELP - get help on commands
- VRFY - verify an address
- EXPN - expand an address
- VERB - verbose
The POP3 Server
In the simplest
implementations of POP3, the server really does maintain a
collection of text files -- one for each e-mail account. When
a message arrives, the POP3 server simply appends it to the
bottom of the recipient's file!
When you check your e-mail, your e-mail client connects to
the POP3 server using port 110. The POP3 server
requires an account name and a password. Once
you have logged in, the POP3 server opens your text file and
allows you to access it. Like the SMTP server, the POP3 server
understands a very simple set of text commands. Here are the
most common commands:
client connects to the POP3 server and issues a series of
commands to bring copies of your e-mail messages to your local machine.
Generally, it will then delete the messages from the server
(unless you've told the e-mail client not to).
- USER - enter your user ID
- PASS - enter your password
- QUIT - quit the POP3 server
- LIST - list the messages and their size
- RETR - retrieve a message, pass it a message
- DELE - delete a message, pass it a message number
- TOP - show the top x lines of a message, pass it
a message number and the number of lines
You can see that the POP3 server simply acts as an
interface between the e-mail client and the text file
containing your messages. And again, you can see that the POP3
server is extremely simple! You can connect to it through
telnet at port 110 and issue the commands yourself if you
would like to (see How Web
Servers and the Internet Work for details on telnetting to
Your e-mail client allows you to
add attachments to e-mail messages you send, and also lets you
save attachments from messages that you receive. Attachments
might include word processing documents, spreadsheets, sound
files, snapshots and pieces of software. Usually, an
attachment is not text (if it were, you would simply include
it in the body of the message). Since e-mail messages can
contain only text information, and attachments are not text,
there is a problem that needs to be solved.
In the early days of e-mail, you solved this problem by
hand, using a program called uuencode. The uuencode
program assumes that the file contains binary information. It
extracts 3 bytes from the binary file and converts them to
four text characters (that is, it takes 6 bits at a time, adds
32 to the value of the 6 bits and creates a text character --
see How Bits
and Bytes Work to learn more about ASCII characters). What
uuencode produces, therefore, is an encoded version of
the original binary file that contains only text characters.
In the early days of e-mail, you would run uuencode yourself
and paste the uuencoded file into your e-mail message.
Here is typical output from the uuencode program:
The recipient would then save the uuencoded
portion of the message to a file and run uudecode on it
to translate it back to binary. The word "reports" in the
first line tells uudecode what to name the output file.
begin 644 reports
M9W)E<" B<&P_(B O=F%R+VQO9R]H='1P9"]W96(V-C1F-
Modern e-mail clients are doing exactly the same thing, but
they run uuencode and uudecode for you automatically. If you
look at a raw e-mail file that contains attachments, you'll
find that the attachment is represented in the same uuencoded
text format shown above!
Considering its tremendous impact on society, having
forever changed the way we communicate, today's e-mail system
is one of the simplest things ever devised! There are parts of
the system, like the routing rules in sendmail, that get
complicated, but the basic system is incredibly
The next time you send an e-mail, you'll know exactly how
it's getting to its destination!
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