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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

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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

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 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?

  • 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: <[email protected]>Delivered-To: [email protected]

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

Received: from

( [])by (Postfix) with ESMTP id 5F41832893 for <[email protected]>; 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 <[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



How SMTP works

  • The Essentials

  • How about a Demo?

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 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 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

TCP Connection Establishment

Message Progress

Connection Termination

TCP Connection Termination

Problems with SMTP

  • No inherent security

    • Authentication

    • Encryption

  • Only uses NVT (Network Virtual Terminal) 7-bit ASCII format

E-mails can be forged…..


MAIL FROM: [email protected]

RCPT TO: [email protected]


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

  • 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

  • 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 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)

  • 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

  • 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)

  • 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.

Next week….Sendmail

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