1. Introduction to Unix
SMTP & Sendmail

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

3. Email Spam Statistics 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

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

9. 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

10. SMTP
SMTP also allows the use of Relays allowing other MTAs to relay the mail
Mail Gateways are used to relay mail prepared by a protocol other then SMTP and convert it to SMTP

11. 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

12. 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

13. 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

14. How SMTP works The Extras Keyword Arguments RSET VRFY
Name to be verified
NOOP
TURN
EXPN
Mailing list to expand
HELP
Command Name

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. Connection Establishment
TCP Connection Establishment

17. Message Progress

18. Connection Termination
TCP Connection Termination

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

20. 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

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

22. MIME Headers Goes between the Email Header and Body MIME-Version: 1.1
Content-Type
Content-Transfer-Encoding
Content-Id
Content-Description

23. MIME Headers
Content-Type – Type of data used in the body of the message
Text – plain, unformatted text; HTML
Multipart – Body contains multiple independent parts
Message – The body is whole mail message, part of a message, or a pointer to a message

24. MIME Headers Image – The message is a stationary image (JPEG or GIF)
Video – The message is an animation (Mpeg)
Audio – The message is 8 kHz standard audio data
Application – The message is a type of data not previously defined

25. MIME Headers
Content-Transfer-Encoding – The method used to encode the messages
7 bit – no encoding needed
8 bit – Non-ASCII, short lines
Binary – Non-ASCII, unlimited length lines
Base64 – 6 bit blocks encoded into 8-bit ASCII
Quoted-printable – send non-ASCII characters as 3 ASCII characters, =##, ## is the hex representation of the byte

26. Base64 Encoding Divides binary data into 24 bit blocks
Each block is then divided into 6 bit chunks
Each 6-bit section is interpreted as one character incurs a 25% overhead
(51)
(8)
(4)
(57)
(z)
(I)
(E)
(5)

27. Quoted-Printable Encoding
Used when the data has a small non-ASCII portion
Non-ASCII characters are sent as 3 characters
First is ‘=‘, second and third are the hex representation of the byte
(=)
(9)
(D)

28. MIME Headers
Content-Id – Uniquely identifies the whole message in a multiple message environment
Content-Description – defines whether the body is image, audio, or video

29. A Multipart, Encoded MIME Message
From: To: Subject: Info on Gibson guitar MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=17
- 17 Content-Type: text/enriched; charset="us-ascii" Content-Transfer-Encoding: 8bit Content-Description: Greetings
As promised, I'm getting back to you about the Gibson Southern Jumbo guitar you were
Interested in. I've enclosed a spec sheet on the guitar, which is in Microsoft Word.
I guarantee that you'll love it!
- 17 Content-Type: application/octet-stream Content-Transfer-Encoding: base64 Content-Description: Spec sheet saved as MS Word file
- 17 -

30. MIME Example Date: Wed, 04 Apr 2001 00:11:37 -0400
From: Meghna Naik
MIME-Version: 1.0
To:
Subject: =?gb2312?B?1tDOxA==?= title
Content-Type: text/plain; charset=gb2312
Content-Transfer-Encoding: 7bit
a body text, blah, blah

31. 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.

32. O’Reilly’s “Bat Book” 3rd Edition

33. This is what happens if you try to learn to much about Sendmail…

34. Sendmail
It's been said that you aren't a real Unix system administrator until you've edited a sendmail.cf file.
It's also been said that you're crazy if you've done it twice.

35. What is Sendmail?
Sendmail is the most widely used Mail Transport Agent (MTA) on the internet
MTAs send mail from one machine to another.
Sendmail is not a client program, which you use to read your .
Sendmail is one of the behind-the-scenes programs which move over the Internet.
Normally it runs as a background daemon
Can even be run out of the super daemon (xinetd)

36. Implementations SMTP Gateway
An SMTP gateway allows users on your network to communicate with others on the Internet without concern as to which local mail software package exists on your network.
All incoming mail for your network will pass through this gateway which converts the message into the appropriate format specific to your local mail software.
Similarly, all mail destined for the Internet from your network will pass through this gateway to be sent across the Internet via SMTP

37. Implemetations SMTP Relay ”Warning Will Rogers”
An SMTP relay is a machine that actually sends the mail across the Internet.
A common misconception is that SMTP gateways are the same as SMTP relays. This is not always the case.
There are SMTP gateways that act as relays themselves, but there are also many that do not. If the latter is the case on your network, you'll need to bounce your mail off one of the relays.

38. The Pieces The binary: The configuration file: Supporting files:
/sbin/sendmail
The configuration file:
/etc/mail/sendmail.cf
Supporting files:
/etc/mail/access
/etc/mail/aliases
…and many more

39. More Pieces Email messages are stored in the directory:
/var/spool/mail
There is a separate file for each user
waiting to be sent
./var/spool/mqueue
A log of sent and received:
/var/log/mail

40. Sendmail Features Sendmail uses DNS (Domain Naming System)
But not 100% dependent:
DNS provides Mail Exchange (MX) Info
Sendmail can do a DNS double-tap
Lookup up who the client says they are
Sendmail default is mail relay off
Realtime Blackhole Lists (RBL)
Mail Relay checkers - - Open Mail Relay Db

41. Sendmail Anti-Spam Enhancements
Mailscanner
Minimal anti-spam
Anti-virus integration (scan in/outbound) Or
Spam Assassin
Rule based heuristic
Header and text analysis
Blacklist (RBL)
Vipul's Razor (

42. Mail Access Protocols The MTAs place the email in the user’s mailbox
The Mail Access Protocols are used by the users to retrieve the from the mailbox
POP3 (Post Office Protocol)
IMAP (Internet Message Access Protocol )

43. POP vs. IMAP POP3: IMAP: …. Whole message headers All Messages
Mr Smith
Friends ….
headers

44. Post Office Protocol v3 Simple
Allows the user to obtain a list of their s
Users can retrieve their s
Users can either delete or keep the on their system
Minimizes server resources

45. Internet Mail Access Protocol v4
Has more features then POP3
User can check the header before downloading
s can be accessed from any location
Can search the for a specific string of characters before downloading
User can download parts of an
User can create, delete, or rename mailboxes on a server

46. References RFC’s: E-mail Explained
RFC Simple Mail Transfer Protocol
RFC Standard for the Format of ARPA Internet Text Messages
RFC MIME (Multipurpose Internet Mail Extensions)
Explained