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Introduction to Unix. SMTP – Simple Mail Transfer Protocol. Overview. Some SPAM Statistics Introduction to SMTP and Email Message Breakdown Sample Messages Extensions (MIME) MTA’s and Mailbox Protocols. 1 st – What is SMTP?.

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introduction to unix

Introduction to Unix

SMTP – Simple Mail Transfer Protocol

overview
Overview
  • Some SPAM Statistics
  • Introduction to SMTP and Email
  • Message Breakdown
  • Sample Messages
  • Extensions (MIME)
  • MTA’s and Mailbox Protocols
1 st what is smtp
1st – What is SMTP?
  • The de facto standard for e-mail transmissions across the Internet
  • It is defined in RFC 821
  • It is a relatively simple, text-based protocol
  • Not entirely secure thus vulnerable to SPAM
email spam statistics 2006
Email Spam Statistics 2006
  • Email considered Spam 40% of all email
  • Daily Spam emails sent 12.4 billion
  • Daily Spam received per person 6
  • Annual Spam received per person 2,200
  • Spam cost to all non-corp Internet users $255 million
  • Spam cost to all U.S. Corporations in 2002 $8.9 billion
  • States with Anti-Spam Laws 26
  • Email address changes due to Spam 16%
  • Estimated Spam increase by 2007 63%
  • Annual Spam in 1,000 employee company 2.1 million
  • Users who reply to Spam email 28%
  • Users who purchased from Spam email 8%
  • Corporate email that is considered Spam 15-20%
  • Wasted corporate time per Spam email 4-5 seconds
slide9
SMTP
  • Originated in 1982 (rfc0821, Jon Postel)
  • Goal: To transfer mail reliably and efficiently
slide10
SMTP
  • SMTP clients and servers have two main components
    • User Agents – Prepares the message, encloses it in an envelope. (Eudora for example)
    • Mail Transfer Agent(MTA) – Transfers the mail across the internet

User Agent

Mail Transfer Agents

slide11
SMTP
  • SMTP also allows the use of Relays allowing other MTAs to relay the mail
what is mail
What is Mail?
  • Mail is a text file
  • Envelope –
    • sender address
    • receiver address
    • other information
  • Message –
    • Mail Header – defines the sender, the receiver, the subject of the message, and some other information
    • Mail Body – Contains the actual information in the message
slide13

Post Office

Mailbox

Return-Path: <[email protected]>Delivered-To: [email protected]

Received: by mail.eecis.udel.edu (Postfix, from userid 62) id 17FBD328DE; Wed, 5 Nov 2003 11:27:02

Received: from mail.acad.ece.udel.edu

(devil-rays.acad.ece.udel.edu [128.4.60.10]) by mail.eecis.udel.edu (Postfix) with ESMTP id 5F41832893 for <[email protected]>; Wed, 5 Nov 2003 11:27:01

Received: by mail.acad.ece.udel.edu (Postfix, from userid 62)id 47509456C; Wed, 5 Nov 2003 11:27:01 Received: from stimpy.eecis.udel.edu(stimpy.eecis.udel.edu [128.4.40.17])by mail.acad.ece.udel.edu (Postfix) with SMTP id 7C2943D79 for <[email protected]>; Wed, 5 Nov 2003 11:26:34 Message-Id: <[email protected]>Date: Wed, 5 Nov 2003 11:26:34 From: [email protected]: undisclosed-recipients: ;MIME-Version: 1.0This is a test.

Post office

and mail route

Receivers

Mailbox

how smtp works
How SMTP works
  • The Essentials
  • How about a Demo?
status codes
Status Codes
  • The Server responds with a 3 digit code that may be followed by text info
    • 2## - Success
    • 3## - Command can be accepted with more information
    • 4## - Command was rejected, but error condition is temporary
    • 5## - Command rejected, Bad User!
status codes1
Status Codes
  • 211 System status, or system help reply .
  • 214 Help message.
  • 220 <domain> Service ready.
  • 221 <domain> Service closing transmission channel.
  • 250 Requested mail action okay, completed.
  • 251 User not local; will forward to <forward-path>.
  • 354 Start mail input; end with <CRLF>.<CRLF>.
  • 421 <domain> Service not available, closing transmission channel. [This may be a reply to any command if the service knows it must shut down].
  • 450 Requested mail action not taken: mailbox unavailable.
  • 451 Requested action aborted: local error in processing
  • 452 Requested action not taken: insufficient system storage.
status codes2
Status Codes
  • 500 Syntax error, command unrecognized. [This may include errors such as command line too long]
  • 501 Syntax error in parameters or arguments.
  • 502 Command not implemented.
  • 503 Bad sequence of commands.
  • 504 Command parameter not implemented.
  • 550 Requested action not taken: mailbox unavailable.
  • 551 User not local; please try <forward-path>.
  • 552 Requested mail action aborted: exceeded storage allocation.
  • 553 Requested action not taken: mailbox name not allowed. [E.g., mailbox syntax incorrect]
  • 554 Transaction failed.
connection establishment
Connection Establishment

TCP Connection Establishment

connection termination
Connection Termination

TCP Connection Termination

problems with smtp
Problems with SMTP
  • No inherent security
    • Authentication
    • Encryption
  • Only uses NVT (Network Virtual Terminal) 7-bit ASCII format
e mails can be forged
E-mails can be forged…..

HELO mail.rose.edu

MAIL FROM: [email protected]

RCPT TO: [email protected]

DATA

From: Dr. Art Zenner

To: Professor Richards

Subject: CIT 2243

Professor Richards,

By department decree all students in your “Introduction to Unix” class are hereby to be granted automatic A’s.

Thank you,

Dr. Art Zenner

.

QUIT

extensions to smtp
Extensions to SMTP
  • MIME – Multipurpose Internet Mail Extensions
    • Transforms non-ASCII data to NVT (Network Virtual Terminal) ASCII data
      • Text
      • Application
      • Image
      • Audio
      • Video
mime and base64 encoding
MIME and Base64 Encoding
  • If the internet is the information highway, then the path for email is a narrow tunnel
    • Only very small vehicles can pass trough
  • Then how do you send a big truck through a small ravine?
    • You have to break it down to smaller pieces and transport the pieces through the ravine, and reassemble the truck
mime and base64 encoding1
MIME and Base64 Encoding
  • The same happens when you send a file attachment via email.
  • This is known as encoding
    • the binary data (256 bits) is transformed to ASCII text (128 bits
    • allowing it to fit through the tunnel
  • On the recipient\'s end, the data is decoded and the original file is rebuilt.
mail transfer agents mta
Mail Transfer Agents (MTA)
  • MTAs do the actual mail transfers
  • MTAs are not meant to be directly accessed by users.
  • Other MTA’s are:
    • Postfix
    • Qmail
    • MS Exchange
    • CC:Mail
    • Lotus Notes
    • ….etc.
problems with simple smtp
Problems with simple SMTP
  • The first one relates to message length. Some older implementations cannot handle messages exceeding 64KB.
  • Another problem relates to timeouts. If the Client and server have different timeouts, one of them may give up while the other is still busy, unexpectedly terminating the connection.
  • Infinite mail storms can be triggered. For example, If host 1 holds mailing list A and host 2 holds mailing list B and each list contains an entry for the other one, then a message sent to either list could generate a never ending amount of email traffic unless somebody checks for it.
esmtp rfc 2821
ESMTP (RFC 2821)
  • To get around the problems with simple SMTP, extended SMTP has been defined in RFC 2821.
  • Clients wanting to use it should send an EHLO message instead of HELO initially. If this is rejected, then the server is a regular SMTP server, and the client should proceed in the usual way. If the EHLO is accepted, then new commands and parameters are allowed.
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