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.
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)
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
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
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
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.
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
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
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?