© 2002 Huhns
SMTP and POP3:
Communicating with
email Servers
Defiana Arnaldy, M.Si
0818 0296 4763
deff_arnaldy@yahoo.com
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•
•
•
•
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Service via Email
Sending Email
SMTP
POP3
Conclusion
© 2002 Huhns
Overview
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• Email is an Internet application that is available
widely: it can be accessed from laptops,
desktops, PDAs, mobile phones, and pagers
• Imagine an application that can receive email
from a user, perform the services requested by
or on behalf of the user, and then send the
results back to the user via email
• The application uses the SMTP and POP3
protocols
© 2002 Huhns
Services via Email
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• Every email must have a destination email address. An email
address takes the following form:
© 2002 Huhns
Sending an email
• The domain name in an email address generally does not
include the “www” prefix, which is common for Web site
addresses
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© 2002 Huhns
• Emails are not immediately delivered to the recipient; instead,
they are initially sent to your ISP’s or company’s mail server.
• Emails are sent using the simple mail transfer protocol (SMTP)
• In order to determine the recipient’s mail server, a DNS mail
exchange (MX) query is issued to the local DNS server for that
domain name
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Electronic Mail
outgoing
message queue
user mailbox
user
agent
Three major components:
 user agents
 mail servers
mail
server
 simple mail transfer protocol:
user
agent
SMTP
mail
server
SMTP
User Agent
SMTP
 a.k.a. “mail reader”
 composing, editing, reading mail
messages
© 2002 Huhns
•
user
agent
SMTP
user
agent
mail
server
 e.g., Eudora, Outlook, elm,
user
agent
Mozilla Thunderbird
 outgoing, incoming messages
stored on server
user
agent
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• Mail Servers
 mailbox contains incoming
messages for user
 message queue of outgoing
(to be sent) mail messages
 SMTP protocol between mail
servers to send email
messages
 client: sending mail server
 “server”: receiving mail
server
user
agent
mail
server
user
agent
SMTP
mail
server
SMTP
© 2002 Huhns
Electronic Mail: mail servers
user
agent
SMTP
user
agent
mail
server
user
agent
user
agent
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Electronic Mail: SMTP [RFC 2821]
port 25
 direct transfer: sending server to receiving server
 three phases of transfer

handshaking (greeting)

transfer of messages

closure
© 2002 Huhns
 uses TCP to reliably transfer email message from client to server,
 command/response interaction

commands: ASCII text

response: status code and phrase
 messages must be in 7-bit ASCII
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• 4) SMTP client sends Alice’s
message over the TCP
connection
• 5) Bob’s mail server places
the message in Bob’s
mailbox
• 6) Bob invokes his user
agent to read message
• 1) Alice uses UA to compose
message and “to”
bob@someschool.edu
• 2) Alice’s UA sends message
to her mail server; message
placed in message queue
• 3) Client side of SMTP
opens TCP connection with
Bob’s mail server
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user
agent
2
mail
server
3
mail
server
4
6
5
© 2002 Huhns
Scenario: Alice sends message to Bob
user
agent
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SMTP
• A sender-SMTP establishes a two-way
transmission channel to a receiver-SMTP on
PORT 25
• HELO command “HELO engr.sc.edu”
• MAIL
command
“MAIL <SP> FROM: address<CRLF>”
• RCPT command “RCPT <SP> TO: address<CRLF>”
• DATA command
• End-of-mail indicator
• QUIT
© 2002 Huhns
Simple Mail Transfer Protocol
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USER
1
Sender – SMTP
mailbox.engr.sc.edu
© 2002 Huhns
Architectural View of SMTP
SMTP Commands/Replies
PORT 25
USER
2
Receiver – SMTP
Hotmail.com
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S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
C:
C:
S:
C:
S:
220 hamburger.edu
HELO crepes.fr
250 Hello crepes.fr, pleased to meet you
MAIL FROM: <alice@crepes.fr>
250 alice@crepes.fr... Sender ok
RCPT TO: <bob@hamburger.edu>
250 bob@hamburger.edu ... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
Do you like ketchup?
How about pickles?
.
250 Message accepted for delivery
QUIT
221 hamburger.edu closing connection
© 2002 Huhns
Sample SMTP interaction
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Mail message format
SMTP: protocol for exchanging
email msgs
• RFC 822: standard for text
message format:
 header lines, e.g.,
 To:
 From:
 Subject:
different from SMTP commands!
 body
 the “message”, ASCII
characters only
header
© 2002 Huhns
•
blank
line
body
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Message format: multimedia extensions
 additional lines in msg header declare MIME content type
 Types: Uuencode, base64, Quoted-printable
MIME version
method used
to encode data
multimedia data
type, subtype,
parameter declaration
encoded data
From: alice@crepes.fr
To: bob@hamburger.edu
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
© 2002 Huhns
 MIME: multimedia mail extension, RFC 2045, 2056
base64 encoded data .....
.........................
......base64 encoded data
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© 2002 Huhns
MIME
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SMTP
SMTP
user
agent
sender’s mail
server
receiver’s mail
server

SMTP: delivery/storage to receiver’s server

Mail access protocol: retrieval from server

access
protocol
user
agent
© 2002 Huhns
Mail access protocols
POP: Post Office Protocol [RFC 1939]
• authorization (agent <-->server) and download

IMAP: Internet Mail Access Protocol [RFC 1730]
• more features (more complex)
• manipulation of stored msgs on server

HTTP: gmail, Hotmail, Yahoo! Mail, etc.
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• telnet servername 25
• see 220 reply from server
• enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands
above lets you send email without using email client
(reader)
© 2002 Huhns
Try SMTP interaction for yourself:
2117
SMTP: final words
Comparison with HTTP:
• HTTP: pull
• SMTP: push
© 2002 Huhns
• SMTP uses persistent
connections
• SMTP requires message
(header & body) to be in 7-bit
ASCII
• SMTP server uses CRLF.CRLF
to determine end of message
• both have ASCII
command/response
interaction, status codes
• HTTP: each object
encapsulated in its own
response msg
• SMTP: multiple objects sent in
multipart msg
2118
POP3
• POP3 is used to receive email messages via a TCP/IP
connection; a client establishes a connection with a POP3
server at PORT 110
© 2002 Huhns
Post-Office Protocol
• Authorization state
USER huhns
PASS *****
• Transaction state
STAT
+OK <SP> # of messages <SP> size of mailbox
• Retrieve command
RETR 45
Response: Header <CRLF> Message content
• DELE msg# & RSET
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• authorization phase
 client commands:
user: declare username
 pass: password
 server responses
 +OK
 -ERR

transaction phase, client:
 list: list message numbers
 retr: retrieve message by
number
 dele: delete
 quit
S:
C:
S:
C:
S:
+OK POP3 server ready
user bob
+OK
pass hungry
+OK user successfully logged
on
© 2002 Huhns
POP3 protocol
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S: <message 1 contents>
S: .
C: dele 1
C: retr 2
S: <message 1 contents>
S: .
C: dele 2
C: quit
S: +OK POP3 server signing off 20
• More about POP3
 Previous example uses
“download and delete”
mode.
 Bob cannot re-read e-mail if
he changes client
 “Download-and-keep”:
copies of messages on
different clients
 POP3 is stateless across
sessions
• IMAP
 Keep all messages in one
place: the server
 Allows user to organize
messages in folders
 IMAP keeps user state
across sessions:
 names of folders and
mappings between
message IDs and folder
name
© 2002 Huhns
POP3 (more) and IMAP
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/*********** After Connection ************/
+OK Microsoft Exchange POP3 server version 5.5.2654.50 ready
/* SERVER */
USER shroff
/* CLIENT */
+OK
/* SERVER */
PASS **********
/* CLIENT */
+OK User successfully logged on
/* SERVER */
STAT
/* CLIENT */
+OK 107 1545500
/* SERVER */
RETR 107
/* CLIENT */
+OK
/* SERVER */
/* HEADER, BLANK LINE, CONTENT */
Received: from imo-m03.mx.aol.com ([64.12.136.6]) by mailbox.engr.sc.edu with SMTP
From: Hm412000@aol.com
Date: Tue, 18 Sep 2001 00:06:54 EDT
Subject: Re: CSCE826 Homework
To: HUHNS@engr.sc.edu
I finished assignment two …
.
© 2002 Huhns
Example POP3 Interaction
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/** PERIOD **/
Example SMTP Interaction
© 2002 Huhns
/*********** After Connection ************/
220 theusc.csd.sc.edu ESMTP Sendmail 8.9.0/8.9.0; Mon, 24 Sep 2001 00:58:05 –0400 /* SERVER */
HELO engr.sc.edu
/* CLIENT */
250 theusc.csd.sc.edu Hello l-1d11-14.engineering [129.252.26.88] pleased to meet you /* SERVER
*/
MAIL FROM: stephens@engr.sc.edu
/* CLIENT */
250 stephens@engr.sc.edu... Sender ok
/* SERVER */
RCPT TO: huhns@engr.sc.edu
/* CLIENT */
250 stephens@engr.sc.edu... Recipient ok
/* SERVER */
Data
/* CLIENT */
354 Enter mail, end with "." on a line by itself
/* SERVER */
Date: Mon Sep 24 01:00:53 EDT 2001
/* CLIENT */
From: stephens@engr.sc.edu Subject: result for your query
any old message
.
250 AAA33192 Message accepted for delivery
QUIT
/* PERIOD Signaling End-of-Message */
/* SERVER */
/* CLIENT */
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© 2002 Huhns
POP3 and SMTP
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Database
Application receives and sends
email
Application
USER
1DB
2Txt File
3Word Doc
4ExcelSheet
5HTML
6Image
7Directory
SpreadSheet
© 2002 Huhns
Service Example: Data and Document Access
Files
Directory
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• The service application receives messages sent by
“USER”
• “USER” specifies file type in SUBJECT field
© 2002 Huhns
Data and Document Access (cont.)
• E.g., DIR, HTML, DB, ALL (for file listing), etc.
• “USER” specifies name of file, query, or command
in CONTENT field
• Application returns results either as email content
or attachment
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• SMTP is used to send emails from client to server.
• POP3 is used to receive emails from server to client.
• POP3 can be used to delete emails from the server once
received.
© 2002 Huhns
Conclusion
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