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Sub Topic : E-Mail Transfer (part 2).

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Recap

In the previous class, you have learnt about

• E-mail

• E-mail transfer mechanism

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Objectives

On the completion of this topic, you would be able to understand

• E-mail Protocols• SMTP• POP

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Electronic mail

Protocols• Simple Mail Transfer Protocol (SMTP) delivers mail to

servers• From clients to local mail server• Inter-mail server delivery

• Post Office Protocol (POP) for user access to delivered email

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Understanding SMTP

• Simple Mail Transfer Protocol (SMTP) is the most widely used protocol for sending electronic mail (e-mail) on the Internet

1

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Basic Email Architecture

Sender client

Recipientclient

Mail Server

Mail Server (relay)

Mail server

SMTP

SMTP SMTP

POP3 or IMAP

SMTP messages are delivered by TCP/IP.

The underlying IP routing generally takes

multiple hops

Mail Server

Mail Server

Fig .2

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Sending Mail with Simple Mail Transport Protocol

• Message gets an SMTP header– Includes host name and timestamp

• TCP Connection on Port 25– Mail client transmits message to its default SMTP

server– Header updated by each mail server handling it– Sender-SMTP server transmits to recipient-SMTP

server– Recipient-SMTP server can forward message to

another mail server

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Electronic Mail: SMTP

• Client’s SMTP mail server establishes a TCP

connection to the recipients SMTP server using

Port 25

• three phases in message transfer

– handshaking (greeting)

– transfer of messages

– closure

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Internet

User Agent User Agent

TCP ephemeral port

Port25

Mail Transfer AgentClient

Mail Transfer AgentServer

SMTP

Fig .3

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Sequence of Events in SMTP

1. TCP connection is established to port 252. The sending machine (client) waits for the receiving

machine (server) to talk first3. The server sends its identity and preparedness to

receive mail4. The client announces whom the e-mail is coming from

and whom it is going to send5. The server gives the client the go-ahead to send the

message6. The client sends the message and the server

acknowledges it7. When all the e-mail has been exchanged in both

directions, the connection is released

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1) Alice uses Mail User Agent (MUA) to compose message and “to” bob@polytechnic.edu

2) Alice’s MUA sends message to her mail server; message placed in message queue

3) Client side of SMTP opens TCP connection with Bob’s mail server

4) SMTP client sends Alice’s message over the TCP connection

5) Bob’s mail server places the message in his mailbox

6) Bob invokes his user agent to read message

useragent

mailserver

mailserver user

agent

1

2 3 4 56

Scenario: Vidya sends message to Rajesh

Alice BobFig .5

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SMTP Elements

• The Sender-SMTP and Receiver-SMTP processes are client or server applications that support SMTP functionality

• Typically the Sender-SMTP transmits a request to initialize an SMTP connection on the Receiver-SMTP server port 25

Sender-SMTP and Receiver-SMTP

• The Sender-SMTP and Receiver-SMTP are processes that support the SMTP commands listed in RFC 821 (SMTP) and RFC 1869

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POP (Post Office Protocol)

• POP stands for Post Office Protocol

• The POP server holds the message until the recipient retrieves it with his/her email software

• POP protocol is used to transfer the mail from POP server to user’s machine

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POP illustration

John’s computerMary’s computer

Mail server

Internet

Mail server

Mailspool

Localinbox

SMTP

SMTPSMTP

POP

Fig .2

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Sequence of Events in POP

1. Alice creates a message for Bob using some e-mail program (i.e., user agent) and clicks “send”

2. The e-mail program hands over the message to the message transfer agent (SMTP Server) on Alice's host

3. The message transfer agent (SMTP Server) sees that it is directed to bob@xyz.com so it uses DNS to look up the MX record for xyz.com. This query returns the DNS name of xyz.com's mail server

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4. The message transfer agent now looks up for the IP address of this machine using DNS again

5. It then establishes a TCP connection to the SMTP server on port 25 of this machine. Using an SMTP command sequence analogous, it transfers the message to Bob mailbox and breaks the TCP connection

Sequence of Events in POP contd…

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Sequence of Events – Contd.

6. Bob boots up his PC, connects to his ISP, and starts his e-mail program

7. The e-mail program establishes a TCP connection to the POP3 server at port 110 of the ISP's mail server machine

8. Bob's e-mail program runs the POP3 protocol to fetch the contents of the mailbox to his hard disk

9. Once all the e-mail has been transferred, the TCP connection is released

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Quiz

1) E-mails are transferred across the internet using___A) Simple Mail Transfer Protocol

2) Messages in SMTP must be in __textA) ASCII

3) SMTP is a ____ protocolA) Push

4) The exchange of mails using TCP/IP is performed by____A) Message Transfer Agent (MTA)

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Quiz

1) The initial stage of POP3 protocol is___

A) Authentication Stage

2) POP uses TCP connector to port ___

A) Number 110

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Frequently Asked Questions

1. List the protocols used for e-mail transfer

2. List the sequence of steps to transfer the email from sender’s SMTP server to receiver’s SMTP server

3. Briefly explain POP protocol