Email i.e. electronic mail , it is a method of exchanging
digital messages from an author to 1 or more recipents .
modern email operates across the internet or other
emails appeared before 1990 but flourished among the public
at large in the 1990's.
What is an E-mail ?
EARLY EMAIL SYSTEMS:
# require the author and the recipient both be online at the
PRESENT EMAIL SYSTEMS:
# based on a store and forward model i.e. neither the users
nor their computers are required to be online simultaneously.
Earlier & present day Email systems:
The first email systems simply consisted of file transfer protocols,
with the convection that the first line of each message contained
the recipient's address.
File Transfer Protocol (FTP) is a standard network protocol
used to transfer files from one host to another host over a TCP-
based network, such as the Internet.
File Transfer Protocol :-
FTP is built on a client-server architecture.
FTP users may authenticate themselves using a clear-text sign-in
protocol, normally in the form of a username and password, but
can connect anonymously if the server is configured to allow it.
Complaints on using FTP systems :
1.Sending a message to a group of people was inconvenient.
Managers often need this
facility to send memos to all their subordinates.
2.Messages had no internal structure, making computer
processing difficult.For example,
in a forwarded message extracting the
forwarded part from the received message was difficult.
3.The originator (sender) never knew if a message arrived or not.
4.It was not possible to create and send messages containing a
mixture of text, drawings , facsimile, and voice.
Present Systems :-
While electronic mail servers and other mail transfer agents use
SMTP to send and receive mail messages, user-level client mail
applications typically only use SMTP for sending messages to a
mail server for relaying.
For receiving messages, client applications usually use either the
Post Office Protocol (POP) or the Internet Message Access
Protocol (IMAP) or a proprietary system (such as Microsoft
Exchange or Lotus Notes/Domino) to access their mail box
accounts on a mail server.
As time passed ,more elaborate email systems were proposed.rfc
821 and rfc 822 technology came to use which were based on
SIMPLE MAIL TRANSFER PROTOCOL i.e. SMTP.8
Architecture And Services :
E-mail systems normally consist of two subsystems:
#The user agents, which allow people to read and send e-mail
#The message transfer agents, which move the messages from
the source to the destination.
The user agents are local programs that provide a command
based , menu-based, or graphical method for interacting with the
The message transfer agents are processes that run in the
Their job is to move e-mail through the system.
Typically, e-mail systems support five basic functions.they are:
Composition refers to the process of creating messages and
For example, when answering a message, the e-mail system can
extract the originator's address from the incoming e-mail and
automatically insert it into the proper place in the reply.
Transfer refers to moving messages from the originator to the
recipient. In large part, this requires establishing a connection to
the destination or some intermediate machine, outputting
the message, and releasing the connection. The e-mail system
should do this automatically,
without bothering the user.
Reporting has to do with telling the originator what happened to
the message. Was it delivered? Was it rejected? Was it lost?
Displaying incoming messages is needed so people can read
Disposition is the final step and concerns what the recipient does
with the message after receiving it. Possibilities include
throwing it away before reading, throwing it away after reading,
saving it, and so on. It should also be possible to retrieve and
reread saved messages, forward them, or process them in other
To explain the architecture of e-mail, we
give four scenarios. We begin with the
simplest situation and add complexity as we
proceed. The fourth scenario is the most
common in the exchange of e-mail.
Topics Discussed HereTopics Discussed Here
When both sender and receiver are
connected to the mail server via a LAN
or a WAN, we need two UAs, two pairs
of MTAs (client and server), and a pair of
MAAs (client and server). This is the
most common situation today.
The first component of an electronic mail
system is the user agent (UA). It provides
service to the user to make the process of
sending and receiving a message easier.
E-mail systems have two basic parts, as we have seen: the user agents and
the message transfer agents.
In this section we will look at the user agents. A user agent is normally a
program (sometimes called a mail reader) that accepts a variety of commands
for composing, receiving, and replying to messages, as well as for manipulating
Some user agents have a fancy menu- or icon-driven interface that requires a
mouse, whereas others expect 1- character commands from the keyboard.
Functionally, these are the same.
Some systems are menu- or icon-driven but also have keyboard shortcuts.
To send an e-mail message, a user must provide the message, the destination
possibly some other parameters. The message can be produced with a free-
editor, a word processing program, or possibly with a specialized text editor
built into the user
agent. The destination address must be in a format that the user agent can
deal with. Many
user agents expect addresses of the form user@dns-address.
This address specifies a country, state, locality, personal address and a
common name . Many other attributes are possible, so you can send e-mail to
someone whose exact email
address you do not know, provided you know enough other attributes (e.g.,
company and job title).
Although X.400 names are considerably less convenient than DNS names,
systems have aliases (sometimes called nicknames) that allow users to enter
or select a
person's name and get the correct e-mail address.
Typically, when a user agent is started up, it looks at the user's
mailbox for incoming e-mail
before displaying anything on the screen. Then it may announce the
number of messages in
the mailbox or display a one-line summary of each one and wait for a
As an example of how a user agent works, let us take a look at a
typical mail scenario. After
starting up the user agent, the user asks for a summary of his e-mail.
Different fields tells different things such as how long the message
is, and one tells who sent the message.
Since this field is simply extracted from the message, this field may
contain first names, full
names, initials, login names, or whatever else the sender chooses
to put there.
Subject field gives a brief summary of what the message is about.
People who fail to include a
Subject field often discover that responses to their e-mail tend not to
get the highest priority.
After the headers have been displayed, the user can perform any of several
actions, such as
displaying a message, deleting a message, and so on. The older systems
were text based and
typically used one-character commands for performing these tasks, such as
T (type message),
A (answer message), D (delete message), and F (forward message). An
the message in question. More recent systems use graphical interfaces.
Usually, the user
selects a message with the mouse and then clicks on an icon to type,
answer, delete, or foward it.
Each header field (logically) consists of a single line of
ASCII text containing the field name, a colon, and, for most
fields, a value.
RFC 822 was designed decades ago and does not clearly
distinguish the envelope fields from the header fields.
The principal header fields related to message transportt
gives the DNS address of the primary recipient. Having
multiple recipients is also allowed. The
RFC 822 :
Cc: field gives the addresses of any secondary
In terms of delivery, there is no
distinction between the primary and secondary
recipients. It is entirely a psychological
difference that may be important to the people
involved but is not important to the mail
Bcc: (Blind carbon copy) field is like the Cc:
this line is deleted from all the copies sent to the
primary and secondary recipients. This
feature allows people to send copies to third
parties without the primary and secondary
recipients knowing this.
RFC 822 header fields related to
message transport :
From and Sender:
tell who wrote and sent the message, respectively. These
need not be the same.
field may be omitted if it is the same as the From: field.
These fields are needed in case the message is
undeliverable and must be returned returned to the
is added by each message transfer agent along the way.
The line contains the agent's identity, the date and time
the message was received, and other information that
can be used for finding bugs in the routing system.
Electronic mail has a simple structure. Its
simplicity, however, comes with a price. It can
send messages only in NVT 7-bit ASCII format.
In other words, it has some limitations.
Multipurpose Internet Mail Extensions (MIME)
is a supplementary protocol that allows non-
ASCII data to be sent through e-mail. MIME
transforms non-ASCII data at the sender site to
NVT ASCII data and delivers it to the client
MTA to be sent through the Internet. The
message at the receiving site is transformed
back to the original data.
Topics Discussed HereTopics Discussed Here
MESSAGE TRANSFER AGENT
The actual mail transfer is done through
message transfer agents (MTAs). To send
mail, a system must have the client MTA, and
to receive mail, a system must have a server
MTA. The formal protocol that defines the
MTA client and server in the Internet is called
Simple Mail Transfer Protocol (SMTP). As we
said before, two pairs of MTA client-server
programs are used in the most common
situation (fourth scenario). Figure 8 shows
the range of the SMTP protocol in this
Topics Discussed HereTopics Discussed Here
Commands and Responses
Mail Transfer Phases
Let us see how we can directly use SMTP to send an
e-mail and simulate the commands and responses we
described in this section. We use TELNET to log into
port 25 (the well-known port for SMTP). We then use
the commands directly to send an e-mail. In this
example, email@example.com is sending an e-
mail to himself. The first few lines show TELNET
trying to connect to the adelphia mail server.
After connection, we can type the SMTP commands
and then receive the responses as shown below. We
have shown the commands in black and the
responses in color. Note that we have added for
clarification some comment lines, designated by the
“=” sign. These lines are not part of the e-mail
MESSAGE ACCESS AGENT
The first and the second stages of mail
delivery use SMTP. However, SMTP is not
involved in the third stage because SMTP is
a push protocol; it pushes the message from
the client to the server. In other words, the
direction of the bulk data (messages) is from
the client to the server. On the other hand,
the third stage needs a pull protocol; the
client must pull messages from the server.
The direction of the bulk data are from the
server to the client. The third stage uses a
message access agent.
Up until now, we have assumed that all users work
on machines that are capable of sending and
receiving e-mail. As we saw, e-mail is delivered by
having the sender establish a TCP connection to the
receiver and then ship the e-mail over it.
However, with the advent of people who access the
Internet by calling their ISP over a modem, it breaks
down. The problem is this: what happens when
Elinor wants to send Carolyn e-mail and Carolyn is
not currently on-line?
Elinor cannot establish a TCP connection to
Carolyn and thus cannot run the SMTP protocol.
One solution is to have a message transfer agent
on an ISP machine accept e-mail for its
customers and store it in their mailboxes on an
ISP machine. Since this agent can be on-line
all the time, e-mail can be sent to it 24 hours a
Unfortunately, this solution creates another
problem: how does the user get the e-mail from
the ISP's message transfer agent? The solution
to this problem is to create another protocol that
allows user transfer agents (on client PCs) to
contact the message transfer agent (on the
ISP's machine) and allow e-mail to be copied
from the ISP to the user. One such protocol is
POP3 (Post Office Protocol Version 3), which
is described in RFC 1939.
Figure 13. (a) Sending and reading mail when the receiver has a
permanent Internet connection and the user agent runs on the same
machine as the message transfer agent. (b) Reading e-mail when the
receiver has a dial-up connection to an ISP.
POP3 begins when the user starts the mail
reader.The mail reader calls up the ISP
(unless there is already a connection) and
establishes a TCP connection with the
message transfer agent at port 110. Once
the connection has been established, the
POP3 protocol goes through three states in
This behavior can be observed by typing something
telnet mail.isp.com 110
where mail.isp.com represents the DNS name of
your ISP's mail server. Telnet establishes a
TCP connection to port 110, on which the POP3
server listens. Upon accepting the TCP
connection, the server sends an ASCII message
announcing that it is present. Usually, it
begins with +OK followed by a comment.
Figure 14. Using POP3 to fetch three messages
disadvantages of pop3
Advantages of POP3:-
Email is available when you are offline
Email is not stored on the server, so
your disk usage on the server is less
Just about any email client (software)
Disadvantages of POP3:-
Can be much slower to check mail
Much harder to do server-side filtering
Mail is inaccessible from other machines
While POP3 allows to access single e-mail account
from work, from their home PC, from their laptop
when on business trips, and from cybercafes when
on so-called vacation, since it normally downloads
all stored messages at each contact, the result is
that the user's e-mail quickly gets spread over
multiple machines, more or less at random, some of
them not even the user's.
This disadvantage gave rise to an alternative final
delivery protocol, IMAP (Internet Message Access
Protocol), which is defined in RFC 2060.
Advantages of IMAP
Email is available from any machine you happen to
Email is stored on the server, so your email cannot
be deleted/destroyed if your computer should
happen to crash, be stolen, or destroyed
You can access IMAP mail via the web, without
even needing a mail client installed. This means you
can check your mail from someone else's machine
or even a public terminal and not have to worry
about the security of your passwords.
Advantages and disadvantages
Some IMAP clients can set up rules for "server side"
filtering. This means that you could put all the emails
from current customers into one mailbox, and filter
other mail (potential new customers) to another
mailbox. This can be done automatically by the
server instead of setting up manual filters in
whatever software you happen to have. This also
means that in most IMAP clients you can subscribe
to only certain mailboxes. For example, at work you
could subscribe to only client mail, at home only to
personal mail, and on your laptop to all your mail. All
with a single account.
If you read a message on one computer, it is
read on any other computer you use to
access your mail. If you reply to an email on
one computer, that reply is available on any
computer you use.
Disadvantages of IMAP:-
Mail is not usually available if you are offline.
An especially valuable feature for many e-mail
users is the ability to set up filters. These are
rules that are checked when e-mail comes in or
when the user agent is started. Each rule specifies
a condition and an action.
Some ISPs provide a filter that automatically
categorizes incoming e-mail as either important
or spam (junk e-mail) and stores each message in
the corresponding mailbox.
Another delivery feature often provided is the ability to
(temporarily) forward incoming e-mail
to a different address. This address can even be a
computer operated by a commercial paging
Still another common feature of final delivery is the
ability to install a vacation daemon. This
is a program that examines each incoming message and
sends the sender an insipid reply such as
Hi. I'm on vacation. I'll be back on the 24th of August. Have a nice
E-mail is such a common application that
some websites today provide this service to
anyone who accesses the site. Three
common sites are Hotmail, Yahoo, and
Google. The idea is very simple. Let us go
through two cases:
Topics Discussed HereTopics Discussed Here
The protocol discussed here does not
provide any security provisions per se.
However, e-mail exchanges can be secured
using two application-layer securities
designed in particular for e-mail systems.
Two of these protocols are Pretty Good
Privacy (PGP) and Secure MIME (SMIME).