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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

  • 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
  • Originated in 1982 (rfc0821, Jon Postel)
  • Goal: To transfer mail reliably and efficiently
  • 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

  • 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

Post Office


Return-Path: <>Delivered-To:

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

Received: from

( []) by (Postfix) with ESMTP id 5F41832893 for <>; Wed, 5 Nov 2003 11:27:01

Received: by (Postfix, from userid 62)id 47509456C; Wed, 5 Nov 2003 11:27:01 Received: from [])by (Postfix) with SMTP id 7C2943D79 for <>; Wed, 5 Nov 2003 11:26:34 Message-Id: <>Date: Wed, 5 Nov 2003 11:26:34 From: Jwatson@cis.udel.eduTo: undisclosed-recipients: ;MIME-Version: 1.0This is a test.

Post office

and mail route



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…..





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



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.