This document discusses electronic mail (email) architecture and protocols. It describes four common email scenarios involving different configurations of users, user agents, mail transfer agents, and mail access agents. It then covers email services provided by user agents, email address formats, MIME for attaching different file types, and protocols like SMTP, POP3, and IMAP4. The most common scenario involves two user agents, two pairs of mail transfer agents, and a pair of mail access agents to allow senders and receivers connected to mail servers via a LAN or WAN to exchange emails.

1. Application Layer
Service:
Electronic Mail

2. Architecture
First Scenario
Simplest
Second Scenario
More Complex
Third Scenario
More Complex
Fourth Scenario
Most common in the exchange of
email

3. Software application that
allows user to read ,compose
and send email.It also handles
mailboxes.
Software application that
transport,deliver and
forward email.
Protocol used to retrieving
incoming mails from local
servers.

4. In the first scenario, the sender and the receiver of the email are users (or
application programs) on the same system

5. In the second scenario, the sender and the receiver of the email are
users (or application programs) on two different systems.

6. In the third scenario,when the sender is connected to the mail server via a
LAN or a WAN,we need two UAs and two pairs of MTAs (client and
server).

7. In the fourth scenario,when both the sender and receiver is connected to the mail
server via a LAN or a WAN, we need two UAs, two pairs of MTAs and a pair of
MAAs.

8. Push and Pull in electronic email

9. Services of User Agent
A user agent
helps the user
compose the e-
mail message to
be sent out.
A user agent read the
incoming messages.
Each email contains
the following fields.
number field, sender,
size of the message,
a flag, subject field
(optional).
After reading a
message, a user can
use the user agent to
reply to a message
to the original
sender or to reply to
all recipients of the
message..
Forwarding is
defined as
sending the
message to a
third party.
Managing
mailbox like
archive a
message or
mark as spam.

10. ● Command-Driven ● GUI-Based Modem
User Agent Types
A command-driven user agent
normally accepts a one-
character command from the
keyboard to perform its task.
Some examples of command-driven
user agents are mail, pine, and
elm.
They contain graphical-user
interface (GUI)components that
allow the user to interact with the
software by using both the keyboard
and the mouse.
Some examples of GUI-based user
agents are Eudora, Microsoft's
Outlook, and Netscape.

11. Format of an e-mail and e-
mail address

12. Multipurpose Internet Mail Extensions MIME
Multipurpose Internet Mail Extensions is a standard to expand limited
capabilities of e-mail.It allows the users to exchange different kinds of data files on
the Internet: audio, video, images, application programs as well.
Features of MIME –
1. It is able to send multiple attachments with a single message.
2. Unlimited message length.
3. Binary attachments (executables, images, audio, or video files) which may be divided if
needed.
4. MIME provided support for varying content types and multi-part messages.

13. Working of MIME

14. MIME header
MIME header is basically inserted at the beginning of any e-mail transfer.

15. Message Transfer Agent: SMTP
The formal protocol that defines the MTA client and server in the Internet is called the Simple
Mail Transfer Protocol (SMTP). Two pairs of MTA client/server programs are used in the
most common situation (fourth scenario).

16. Commands and Responses
Command
format
SMTP uses commands and responses to transfer messages between an
MTA client and an MTA server.

17. Commands Response

18. Message Access Protocols
Currently two message access protocols are available:
1) Post Office Protocol, version 3 (POP3)
2) Internet Mail Access Protocol, version 4 (IMAP4).

19. Presentation By:
Amisha Sahu