This document discusses electronic mail representation and transfer. It describes the user agent, which prepares messages and stores them in mailboxes. Message transfer agents like SMTP transfer messages between systems using commands and responses. POP3 and IMAP are message access agents that allow users to retrieve messages stored on mail servers through protocols like POP3 or IMAP. MIME extensions allow non-text content like images and files to be sent via email.

1. Lecture 9: Electronic Mail Representation and Transfer
Electronic Mail Paradigm, SMTP, POP/ IMAP

2. Outline
 Introduction
 Architecture
 User Agent
 Message Transfer Agent: SMTP
 Message Access Agent: POP and IMAP
 Web-Based Mail

3. Electronic Mail
 At beginning email were short and text
only, today email much more complex.
 It allows a message to include text, audio,
and video.
 It also allows one message to be sent to
one or more recipients

4. Architecture
 To explain the architecture of e-mail we will
use four scenarios

5. First scenario in electronic mail
Alice (user) send a message to Bob (another user) using User Agent
program in order to prepare the message and store it in Bob’s Mailbox.
The message has sender’s and recipient mailbox address (names of files)
Bob can retrieve and read the contents on his mailbox at any time using User
Agent

6. First scenario
 Sender and receiver of email are users on the
same system (mail server).
 They directly connected to a shared system
 Administrator created one mailbox for each user
where the received messages are stored
 A Mailbox is part of local hard drive – a special
file with permission restriction. Only owner of
mailbox has access to the mailbox

7. Second scenario in electronic mail
Alice needs to use UA program to send her message to the system (mail server)
at her own site. Mail server at her site uses queue to store messages waiting to be
sent.
Bob also needs a UA to retrieve messages stored in the mailbox of the mail server
at his site
Message needs to be sent through the internet from Alice’s site to Bob’s site.
So two MTAs are needed. MTA Client and MTA server

8. Second scenario
 Sender and receiver of the email are users
on two different system
 Message needs to be sent over the Internet
 Here we need to have User Agents (UA)
and Message Transfer Agents (MTA)

9. Third scenario in electronic mail
Bob as in 2nd scenario.
Alice is separated from her
mail server, connected to
server through LAN or WAN
Alice still need to use UA to
prepare the message. Then
she can send it through
LAN/WAN to MTA

10. Fourth scenario in electronic mail
This is most common
scenario.
Bob and Alice are
connected to Mail server
through LAN/WAN.
Here we need a set of
client/server agent,
Message Access Agent
(MAA)

11. User Agent

12. User Agent
 Compose Message
 To write message in proper format
 Provide template, built-in editor that can spell checking,
grammar checking etc.
 Read Message
 Read incoming message
 Each received message contain;
 Number field
 Flag – status of email. E.g. new, read
 Size of message
 Sender
 Optional subject field

13. User Agent
 Reply Message
 Reply to the sender
 Forward Message
 Send the message to third party
 Handle Mailbox
 UA usually has Inbox and Outbox
 Inbox – keeps all received emails
 Outbox – keeps all sent email

14. User Agent
 Command Driven
 Accepts one character command from keyboard
to perform its task. E.g. r to reply
 Examples: mail, pine, and elm
 GUI- based
 Microsoft Outlook, Netscape, Eudora,
Thunderbird

15. User Agent

16. Sending email
Format of an e-mail

17. Message
 Consist of Header and Body
 Header
 Define sender, subject of message and any
other information
 Body
 Contain the actual information to be read by
receiver

18. Receiving Mail
 UA is triggered by user (or timer)
 If user has mail, UA inform user with notice.
 If user ready to read the mail a list is
displayed in which each line contain
summary of information
 User can select any message and displays
its contents on the screen.

19. E-mail address
Local part: define name of special file (user mailbox)
where all mail received for user is stored for retrieval
by MAA
Domain name: host to receive and send e-mail. Mail
server of exchanger

20. Mailing List
 Allow one name (alias) to represent
different e-mail addresses.
 Every time message to be sent system
checks the recipient name against alias
database

21. MIME – Multipurpose Internet Mail
Extensions
 E-mail can only send message in NVT 7-bit
ASCII.
 MIME – supplementary protocol to allow
non ASCII data sent through e-mail.
 Allows transmission of
 Binary data
 Multimedia files (video/audio clips)
 Multiple types in single message
 Mixed formats

22. MIME

23. MIME header

24. Data types and subtypes in MIME

25. Message Transfer Agent: SMTP
SMTP – define how commands and responses must be sent back
and forth

26. SMTP - Simple Mail Transfer Protocol
 Runs over TCP
 Used between
 Mail transfer program on sender’s computer
 Mail server on recipient’s computer
 Specifies how
 Client interacts with server
 Recipients specified
 Message is transferred

27. SMTP
 SMTP is simple ASCII protocol where
 sender makes TCP connection to port 25 and
waits
 receiver identifies itself and says if it will receive
e-mail
 if rejected, sender tries again later

28. SMTP: Commands and responses
Command Format
SMTP uses commands and responses to transfer messages
between MTA client and MTA server

29. SMTP: Commands

30. SMTP: Responses

31. SMTP: Responses (cont.)

32. SMTP - example
 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, forouzanb@adelphia.net 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 on the next slide.
 Note that we have added, for clarification, some comment lines,
designated by the “=” signs. These lines are not part of the e-
mail procedure.

33. SMTP session
$ telnet mail.adelphia.net 25
Trying 68.168.78.100 . . .
Connected to mail.adelphia.net (68.168.78.100).

34. SMTP session

35. Message Access Agent: POP3 and IMAP4

36. POP3 and IMAP4
 First and second stages of mail delivery use
SMTP. STMP not involve in the third stage
because it is a push protocol; client push
message to server
 The third stage needs a pull protocol;
client pull the message from server
 POP3 – Post Office Protocol ver. 3
 IMAP4 – Internet Mail Access Protocol ver. 4

37. Push versus pull in electronic email

38. POP3
 POP3 client is installed in recipient computer.
 POP3 server is installed in mail server
 Mail access start when a client downloads its e-
mail messages from a server.
 Two modes
 Delete: mail is deleted from mailbox after retrieval
 Keep: mail remains in mailbox after retrieval

39. The exchange of commands and
responses in POP3
Client opens a connection to
server on TCP port 110.
Then it send user name and
password to access mailbox.
After that, user can list and
retrieve mail message.

40. IMAP4
 IMAP4 is more sophisticated protocol that
offers
 Check email header prior to downloading
 Search the content of email for specific string
 Partially download the email
 Create, delete or rename mailbox on mail server
 Create hierarchy of mailboxes in a folder for
email storage

41. Web-based Mail
 Yahoo Mail, Gmail, Hotmail and etc..
 Mail transfer from sender’s browser to mail
server through HTTP.
 Transfer of message from sending mail
server to receiving server still through SMTP
 Message from receiving server (web server)
to receiver’ browser is done through HTTP
 Instead of POP3 and IMAP4, HTTP is used
as MAA

42. Web-based Mail

43. Summary
 Several program (protocol) including SMTP, POP3 and IMAP4, are used
in the Internet to provide e-mail service
 The UA prepares the message, creates the envelope, and puts the
message in the envelope.
 The email address consists of two parts: a local address (user mailbox)
and a domain name. The form is localname@domainname.
 The MTA transfers the email across the Internet, LAN or WAN.
 SMTP uses commands and responses to transfer messages between an
MTA client and an MTA server.
 The steps in transferring a mail message are connection establishment,
message transfer, and connection termination.
 POP3 and IMAP4 used for pulling messages from mail server
 Multipurpose Internet Mail Extension (MIME) is an extension of SMTP
that allows the transfer of multimedia and other non-ASCII messages.

44. Review Questions
 Describe the addressing system used by
SMTP
 In e-mail, what are the tasks of User agent?
 What is MIME?
 Why do we need POP3 or IMAP4 for e-mail?