SMTP and POP are the main protocols used to transfer emails. SMTP is used to deliver emails from client to local mail server and between mail servers, while POP is used for user access to delivered emails. SMTP uses TCP connections on port 25 to transfer messages in three phases - handshaking, transfer, and closure. POP retrieves emails from the POP server to the user's machine using a TCP connection on port 110 in two stages - authentication and message transfer.

1. Sub Topic : E-Mail Transfer (part 2)
.
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2. Recap
In the previous class, you have learnt about
• E-mail
• E-mail transfer mechanism
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3. Objectives
On the completion of this topic, you would be
able to understand
• E-mail Protocols
• SMTP
• POP
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4. 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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5. Understanding SMTP
• Simple Mail Transfer Protocol (SMTP) is the most widely
used protocol for sending electronic mail (e-mail) on the
Internet
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6. Basic Email Architecture
SMTP messages are
Sender delivered by TCP/IP.
client The underlying IP
routing generally takes
SMTP multiple hops
SMTP Mail SMTP
Mail Mail
Server
Server server
Mail (relay)
Server
Mail
Server
POP3 or IMAP
Recipient
client Fig .2
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7. 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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8. 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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9. SMTP
User Agent User Agent
Mail Transfer Agent Mail Transfer Agent
Client Server
Port
TCP ephemeral 25
port
Internet
Fig .3
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10. Sequence of Events in SMTP
1. TCP connection is established to port 25
2. The sending machine (client) waits for the receiving
machine (server) to talk first
3. The server sends its identity and preparedness to
receive mail
4. The client announces whom the e-mail is coming from
and whom it is going to send
5. The server gives the client the go-ahead to send the
message
6. The client sends the message and the server
acknowledges it
7. When all the e-mail has been exchanged in both
directions, the connection is released
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11. Scenario: Vidya sends message to Rajesh
1) Alice uses Mail User Agent 4) SMTP client sends Alice’s
(MUA) to compose message message over the TCP
and “to” connection
bob@polytechnic.edu
5) Bob’s mail server places the
2) Alice’s MUA sends message message in his mailbox
to her mail server; message
6) Bob invokes his user agent to
placed in message queue
read message
3) Client side of SMTP opens
TCP connection with Bob’s
mail server
1 mail
mail
server user
user server
2 agent
agent 3 4 6
5
Fig .5 Bob
Alice
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12. 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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13. 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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14. POP illustration
Mary’s computer
John’s computer
Local
inbox
SMTP Internet POP
SMTP SMTP
Mail
spool
Mail server Mail server
Fig .2 14

15. 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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16. Sequence of Events in POP contd…
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
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17. 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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18. Quiz
1) E-mails are transferred across the internet using___
A) Simple Mail Transfer Protocol
2) Messages in SMTP must be in __text
A) ASCII
3) SMTP is a ____ protocol
A) Push
4) The exchange of mails using TCP/IP is performed by____
A) Message Transfer Agent (MTA)
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19. 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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20. 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
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