1. SMTP – Simple Mail Transfer Protocol
Introduction to Unix
SMTP – Simple Mail Transfer Protocol

2. Overview Some SPAM Statistics Introduction to SMTP and Email
Message Breakdown
Sample Messages
Extensions (MIME)
MTA’s and Mailbox Protocols

3. 1st – What is SMTP?
The de facto standard for 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

4. Email Spam Statistics 2006 Email considered Spam 40% of all email
Daily Spam s 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
address changes due to Spam 16%
Estimated Spam increase by %
Annual Spam in 1,000 employee company 2.1 million
Users who reply to Spam 28%
Users who purchased from Spam 8%
Corporate that is considered Spam 15-20%
Wasted corporate time per Spam 4-5 seconds

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

10. 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

11. SMTP
SMTP also allows the use of Relays allowing other MTAs to relay the mail

12. What is Mail? Mail is a text file Envelope – Message – 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

13. Post Office
Mailbox
Return-Path: Delivered-To:
Received: by mail.eecis.udel.edu (Postfix, from userid 62) id 17FBD328DE; Wed, 5 Nov :27:02
Received: from mail.acad.ece.udel.edu
(devil-rays.acad.ece.udel.edu [ ]) by mail.eecis.udel.edu (Postfix) with ESMTP id 5F for Wed, 5 Nov :27:01
Received: by mail.acad.ece.udel.edu (Postfix, from userid 62)id C; Wed, 5 Nov :27:01 Received: from stimpy.eecis.udel.edu(stimpy.eecis.udel.edu [ ])by mail.acad.ece.udel.edu (Postfix) with SMTP id 7C2943D79 for Wed, 5 Nov :26:34 Message-Id: Date: Wed, 5 Nov :26:34 From: To: undisclosed-recipients: ; MIME-Version: 1.0 This is a test.
Post office
and mail route
Receivers
Mailbox

14. How SMTP works The Essentials How about a Demo? Keyword Arguments HELO
Sender’s Host Domain Name
MAIL FROM:
Address of sender
RCPT TO:
of Intended recipient
DATA
Body of the message
QUIT

15. 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!

16. 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.

17. 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.

18. Connection Establishment
TCP Connection Establishment

19. Message Progress

20. Connection Termination
TCP Connection Termination

21. Problems with SMTP No inherent security
Authentication
Encryption
Only uses NVT (Network Virtual Terminal) 7-bit ASCII format

22. E-mails can be forged….. HELO mail.rose.edu
MAIL FROM:
RCPT TO:
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

23. Extensions to SMTP MIME – Multipurpose Internet Mail Extensions
Transforms non-ASCII data to NVT (Network Virtual Terminal) ASCII data
Text
Application
Image
Audio
Video

24. MIME and Base64 Encoding
If the internet is the information highway, then the path for 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

25. MIME and Base64 Encoding
The same happens when you send a file attachment via .
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.

26. 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.

27. 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 traffic unless somebody checks for it.

28. 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.

29. Next week….Sendmail