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Introduction to E-mail

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 article, we'll take an in-depth look at e-mail and how it works.

An E-mail Message

According to Darwin Magazine: Prime Movers, 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.

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:

• 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.

• 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 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.

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.

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:

• 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 ravish, Aman Smith's might be aman, 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 RAVISH.TXT, another named AMAN.TXT, and so on.

• If someone wanted to send me a message, the person would compose a text message ("Ravish, Can we have lunch Monday? Aman") in an e-mail client, and indicate that the message should go to ravish. 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 (ravish), the name of the sender (aman) and the body of the message.

• The server would format those pieces of information and append them to the bottom of the RAVISH.TXT file. The entry in the file might look like this:

From: aman
To: Rani
Can we have lunch Monday?

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.

How it works?

From the user standpoint, email seems so simple. You select the address of the person to whom you want to send the email, compose your message and click 'Send'.

All done.

In reality, sending your message off into the network cloud is a bit like sending Little Red Riding Hood into the deep dark woods. You never know what might happen.


[Attact the SWF here]

New Terms:

MUA: Mail user Agent

MTA: Mail Transfer Agent

MDA: Mail Delivery Agent

How email really works

In this diagram, the sender is a human being using their company account to send an email to someone at a different company.

Step A: Sender creates and sends an email

The originating sender creates an email in their Mail User Agent (MUA) and clicks 'Send'. The MUA is the application the originating sender uses to compose and read email, such as Eudora, Outlook, etc.

Step B: Sender's MDA/MTA routes the email

The sender's MUA transfers the email to a Mail Delivery Agent (MDA). Frequently, the sender's MTA also handles the responsibilities of an MDA. ;

The MDA/MTA accepts the email, then routes it to local mailboxes or forwards it if it isn't locally addressed.

In our diagram, an MDA forwards the email to an MTA and it enters the first of a series of "network clouds," labeled as a "Company Network" cloud.

Step C: Network cloud

An email can encounter a network cloud within a large company or ISP, or the largest network cloud in existence: the Internet. The network cloud may encompass a multitude of mail servers, DNS servers, routers, lions, tigers, bears (wolves!) and other devices and services too numerous to mention. These are prone to be slow when processing an unusually heavy load, temporarily unable to receive an email when taken down for maintenance, and sometimes may not have identified themselves properly to the Internet through the Domain Name System (DNS) so that other MTAs in the network cloud are unable to deliver mail as addressed. These devices may be protected by firewalls, spam filters and malware detection software that may bounce or even delete an email. When an email is deleted by this kind of software, it tends to fail silently, so the sender is given no information about where or when the delivery failure occurred.

Email service providers and other companies that process a large volume of email often have their own, private network clouds. These organizations commonly have multiple mail servers, and route all email through a central gateway server (i.e., mail hub) that redistributes mail to whichever MTA is available. Email on these secondary MTAs must usually wait for the primary MTA (i.e., the designated host for that domain) to become available, at which time the secondary mail server will transfer its messages to the primary MTA.

Step D: Email queue

The email in the diagram is addressed to someone at another company, so it enters an email queue with other outgoing email messages. If there is a high volume of mail in the queue—either because there are many messages or the messages are unusually large, or both—the message will be delayed in the queue until the MTA processes the messages ahead of it.

Step E: MTA to MTA transfer

When transferring an email, the sending MTA handles all aspects of mail delivery until the message has been either accepted or rejected by the receiving MTA.

As the email clears the queue, it enters the Internet network cloud, where it is routed along a host-to-host chain of servers. Each MTA in the Internet network cloud needs to "stop and ask directions" from the Domain Name System (DNS) in order to identify the next MTA in the delivery chain. The exact route depends partly on server availability and mostly on which MTA can be found to accept email for the domain specified in the address. Most email takes a path that is dependent on server availability, so a pair of messages originating from the same host and addressed to the same receiving host could take different paths. These days, it's mostly spammers that specify any part of the path, deliberately routing their message through a series of relay servers in an attempt to obscure the true origin of the message.

To find the recipient's IP address and mailbox, the MTA must drill down through the Domain Name System (DNS), which consists of a set of servers distributed across the Internet. Beginning with the root nameservers at the top-level domain (.tld), then domain nameservers that handle requests for domains within that .tld, and eventually to nameservers that know about the local domain.

DNS resolution and transfer process

• There are 13 root servers serving the top-level domains (e.g., .org, .com, .edu, .gov, .net, etc.). These root servers refer requests for a given domain to the root name servers that handle requests for that tld. In practice, this step is seldom necessary.

• The MTA can bypass this step because it has already knows which domain name servers handle requests for these .tlds. It asks the appropriate DNS server which Mail Exchange (MX) servers have knowledge of the subdomain or local host in the email address. The DNS server responds with an MX record: a prioritized list of MX servers for this domain.

An MX server is really an MTA wearing a different hat, just like a person who holds two jobs with different job titles (or three, if the MTA also handles the responsibilities of an MDA). To the DNS server, the server that accepts messages is an MX server. When is transferring messages, it is called an MTA.

• The MTA contacts the MX servers on the MX record in order of priority until it finds the designated host for that address domain.

• The sending MTA asks if the host accepts messages for the recipient's username at that domain (i.e., username@domain.tld) and transfers the message.

Step F: Firewalls, spam and virus filters

The transfer process described in the last step is somewhat simplified. An email may be transferred to more than one MTA within a network cloud and is likely to be passed to at least one firewall before it reaches it's destination.

An email encountering a firewall may be tested by spam and virus filters before it is allowed to pass inside the firewall. These filters test to see if the message qualifies as spam or malware. If the message contains malware, the file is usually quarantined and the sender is notified. If the message is identified as spam, it will probably be deleted without notifying the sender.

Spam is difficult to detect because it can assume so many different forms, so spam filters test on a broad set of criteria and tend to misclassify a significant number of messages as spam, particularly messages from mailing lists. When an email from a list or other automated source seems to have vanished somewhere in the network cloud, the culprit is usually a spam filter at the receiver's ISP or company. This explained in greater detail in Virus Scanning and Spam Blocking.


In the diagram, the email makes it past the hazards of the spam trap...er...filter, and is accepted for delivery by the receiver's MTA. The MTA calls a local MDA to deliver the mail to the correct mailbox, where it will sit until it is retrieved by the recipient's MUA.

SMTP Server

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:

1. Ask the server to send a copy of the MBRAIN.TXT file

2. Ask the server to erase and reset the MBRAIN.TXT file

3. Save the MBRAIN.TXT file on my local machine

4. Parse the file into the separate messages (using the word "From:" as the separator)

5. 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.

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 ().

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 e-mail.


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 Rani@mindspring.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:

1. Outlook Express connects to the SMTP server at mail.howstuffworks.com using port 25.

2. 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 message.

3. The SMTP server takes the "to" address (Rani@mindspring.com) and breaks it into two parts:

• The recipient name (Rani)

• The domain name (mindspring.com)

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.

4. The SMTP server has a conversation with a Domain Name Server, or DNS (see How Web Servers 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.

5. 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 HowStuffWorks, and gives the message to the Mindspring server. The Mindspring server recognizes that the domain name for Rani is at Mindspring, so it hands the message to Mindspring's POP3 server, which puts the message in Rani's mailbox.

If, for some reason, the SMTP server at HowStuffWorks 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.

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 like this:

hello test
250 mx1.mindspring.com Hello abc.sample.com
[], pleased to meet you
mail from: test@sample.com
250 2.1.0 test@sample.com... Sender ok
rcpt to: Rani@mindspring.com
250 2.1.5 Rani... Recipient ok
354 Enter mail, end with "." on a line by itself
from: test@sample.com
subject: testing
Aman, I am testing...
250 2.0.0 e1NMajH24604 Message accepted
for delivery
221 2.0.0 mx1.mindspring.com closing connection
Connection closed by foreign host.

What the e-mail client says is in blue, 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 "spoof" e-mail.

You can see that the SMTP server understands very simple text commands like HELO, MAIL, RCPT and DATA. The most common commands are:

• HELO - introduce yourself
• EHLO - introduce yourself and request extended mode
• 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

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:

• 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 number

• 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

Your e-mail 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).

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 ( ).

The IMAP Server

As you can see, the POP3 protocol is very simple. It allows you to have a collection of messages stored in a text file on the server. Your e-mail client (e.g. Outlook Express) can connect to your POP3 e-mail server and download the messages from the POP3 text file onto your PC. That is about all that you can do with POP3.

Many users want to do far more than that with their e-mail, and they want their e-mail to remain on the server. The main reason for keeping your e-mail on the server is to allow users to connect from a variety of machines. With POP3, once you download your e-mail it is stuck on the machine to which you downloaded it. If you want to read your e-mail both on your desktop machine and your laptop (depending on whether you are working in the office or on the road), POP3 makes life difficult.

IMAP (Internet Mail Access Protocol) is a more advanced protocol that solves these problems. With IMAP, your mail stays on the e-mail server. You can organize your mail into folders, and all the folders live on the server as well. When you search your e-mail, the search occurs on the server machine, rather than on your machine. This approach makes it extremely easy for you to access your e-mail from any machine, and regardless of which machine you use, you have access to all of your mail in all of your folders.

IMAP Problems and Attachments

Your e-mail client connects to the IMAP server using port 143. The e-mail client then issues a set of text commands that allow it to do things like list all the folders on the server, list all the message headers in a folder, get a specific e-mail message from the server, delete messages on the server or search through all of the e-mails on the server.

One problem that can arise with IMAP involves this simple question: "If all of my e-mail is stored on the server, then how can I read my mail if I am not connected to the Internet?" To solve this problem, most e-mail clients have some way to cache e-mail on the local machine. For example, the client will download all the messages and store their complete contents on the local machine (just like it would if it were talking to a POP3 server). The messages still exist on the IMAP server, but you now have copies on your machine. This allows you to read and reply to e-mail even if you have no connection to the Internet. The next time you establish a connection, you download all the new messages you received while disconnected and send all the mail that you wrote while disconnected.


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 (). 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:

begin 644 reports
M9W)E<" B<&P_(B O=F%R+

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.

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 straightforward.

The next time you send an e-mail, you'll know exactly how it's getting to its destination.

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