This document discusses email and how it works. There are three main components: user agents like Outlook or Thunderbird, mail servers, and SMTP (Simple Mail Transfer Protocol). SMTP is used to transfer emails between mail servers over TCP. It uses commands and responses to reliably transfer messages. Email addresses contain the server name and user name. User agents are used to compose and read emails, which are stored on mail servers in user mailboxes.

1. Computer Networks – CSE331
Lecture 9
2 Hr

2. Three major components:
 user agents
 mail servers
 simple mail transfer protocol: SMTP
User Agent
 “mail reader”
 composing, editing, reading mail
messages
 e.g., Outlook, Thunderbird
 outgoing, incoming messages stored on
server
Email Address has two parts
- Server name
- user name on that server
user mailbox
outgoing
message queue
mail
server
mail
server
mail
server
SMTP
SMTP
SMTP
user
agent
user
agent
user
agent
user
agent
user
agent
user
agent
1980’s – before HTTP

3.  uses TCP to reliably transfer email message from client to
server, port 25
 direct transfer: sending server to receiving server
 three phases of transfer
 handshaking (greeting)
 transfer of messages
 closure
 command/response interaction (like HTTP, FTP)
 commands: ASCII text
 response: status code and phrase
 messages must be in 7-bit ASCII
 Push protocol
 Multiple objects as compared to HTTP

4. user
agent
1) A uses UA to compose
message “to” b@gmail.com
2) A’s UA sends message to his
mail server; message placed
in message queue
3) Mail server check that b is in
the same server
4) Mail is transferred to B’s
mail box
5) B invokes his user agent
to read message
mail
server
1
2 3
5
A’s mail server
B’s mail server
user
agent
a@gmail.com b@gmail.com
4

5. user
agent
1) A uses UA to compose
message “to” b@yahoo.com
2) A’s UA sends message to his
mail server; message placed
in message queue
3) client side of SMTP opens
TCP connection with B’s
mail server
4) SMTP client sends A’s
message over the TCP
connection
5) B’s mail server places the
message in B’s mailbox
6) B invokes his user agent
to read message
mail
server
mail
server
1
2 3 4
5
6
gmail server yahoo mail server
user
agent
a@gmail.com
b@yahoo.com

6. connection b/w mail client and
mail server
220 yahoo.com
HELO gmail.com
250 Nice
MAIL FROM: <a@gmail.com>
250 OK
RCPT TO: <b@yahoo.com>
250 OK
DATA
354 Send data
Hi, how are you?
221 Service closing
Mail
Server
Mail
Client
I am coming this Monday.
.
250 Accepted
QUIT
Each line ends with
CRLF
To end email data
single dot & CRLF
Now a days most
servers blocked port
25.
SSL 465 TSL 587
Try telnet mta7.am0.yahoodns.net 25 OR telnet alt4.gmail-smtp-in.l.google.com 25
OR telnet smtp.gmail.com 25

8. And much more … must see for understanding SMTP headers
 https://mediatemple.net/community/products/dv/204643950/understanding-an-
email-header
 https://www.howtogeek.com/108205/htg-explains-what-can-you-find-in-an-email-
header/
Header Description & Example
From Let the receiver know sender
From: Ali<Ali37483@gmail.com>
To Receiver mail address
To: receiver@someserver.com
Date Add email date
Date: Thu, 26 Feb 2017 09:27:04
Subject Let receiver know subject of email
Subject: First Aid Workshop

9. connection b/w mail client and
mail server
220 yahoo.com
HELO gmail.com
250 Nice
MAIL FROM: <a@gmail.com>
250 OK
RCPT TO: <b@yahoo.com>
250 OK
DATA
354 Send data
From: a@gmail.com
Mail
Server
Mail
Client
To: b@gmail.com
Date: Thu, 26 Feb 2017 09:27:04
Subject: Holidays
Header end with
empty line i.e CRLF
Hi, how are you?
221 Service closing
I am coming this Monday.
.
250 Accepted
QUIT
See reply codes 23.5 table
Forouzan book

10.  Great we have sent the emails, time to read them
 Pull protocol needed to read emails
 Login to your server and manage your emails
 Post office protocol 2 (POP3) – Port 110
 Internet mail access protocol (IMAP) – Port 143
mail
server
B’s mail server
user
agent
b@gmail.com

11.  Let the user download emails to his system
 Firs authorize
Command Argument Description
USER User id User name to login
PASS Password Login password
LIST Get list of emails, Serial no and email
length returned
RETR Email no Download given email
DELE Email no Delete email from server disk
QUIT Sign off

12. connection b/w mail client and
mail server 110 port
+OK ready
user username
+Ok
pass password
+Ok Logged in
LIST
+Ok 10 messages
1 1909
Mail
Server
Mail
Client
Dele 2
+Ok logging out
+Ok 2 deleted
QUIT
retr 1
…
+Ok 1909 octecs
To: …
…
Success returned with +Ok error
with -ERR

13.  You are saving some draft
 Move emails to different folders etc
 Search emails
 Partial content transfer
 etc

14.  Users access mail boxes via web page
 Sending email from user to mail server is just like posting a
simple form
 Server uses SMTP to deliver emails to other servers
 But client can still use SMTP
 Any one used outlook mail client?
 Remember
 All the things were in plain text, no security
 Some servers block non-secure ports
 New RFC defines new Ports over SSL/TSL to secure the communication
 Demo

15.  Ross book 2.4
 Forouzan book Chapter 23 (Leave details of MIME)
 SMTP header analysis – Tracing email origin
 https://mediatemple.net/community/products/dv/204643950/understanding-an-
email-header
 https://www.howtogeek.com/108205/htg-explains-what-can-you-find-in-an-email-
header/