1. Internet Engineering Course
Electronic Mail

2. Electronic Mail Most heavily used application on any network
Uses client-server architecture
Electronic mail client accepts mail from user and delivers to server on destination computer
Many variations and styles of delivery
Simple Mail Transfer Protocol (SMTP)
TCP/IP
Delivery of simple text messages
Multi-purpose Internet Mail Extension (MIME)
Delivery of other types of data
Voice, images, video clips

3. SMTP Concept

4. User Agents and Mail Transfer Agents

5. Relay MTAs

6. Mail Gateways

7. The Entire E-mail System

8. SMTP
Simple Mail Transfer Protocol is standard application protocol for delivery of mail from source to destination
Built on top of TCP: provides reliable delivery of data
SMTP does not normally use intermediate mail servers for sending mail
SMTP uses persistent connections
Other functions:
address lookup
address verification
Example protocol exchange

9. Sample SMTP interaction
c:\Telnet payk.ui.ac.ir 25

11. SMTP RFC 821 Not concerned with format of messages or data
Covered in RFC 822 (see later)
SMTP uses info written on envelope of mail
Message header
Does not look at contents
Message body
Except:
Standardize message character set to 7 bit ASCII

12. Basic Operation Mail created by user agent program (mail client)
Message consists of:
Header containing recipient’s address and other info
Body containing user data
Messages queued and sent as input to SMTP sender program
Typically a server process (daemon on UNIX)

13. Mail Message Contents Each queued message has:
Message text
RFC 822 header with message envelope and list of recipients
Message body, composed by user
A list of mail destinations
Derived by user agent from header
May be listed in header
May require expansion of mailing lists
May need replacement of mnemonic names with mailbox names
If BCCs indicated, user agent needs to prepare correct message format

14. SMTP Sender Takes message from queue
Transmits to proper destination host
Via SMTP transaction
Over one or more TCP connections to port 25
Host may have multiple senders active
When delivery complete, sender deletes destination from list for that message
When all destinations processed, message is deleted

15. Optimization
If message destined for multiple users on a given host, it is sent only once
Delivery to users handled at destination host
If multiple messages ready for given host, a single TCP connection can be used
Saves overhead of setting up and dropping connection

16. Possible Errors Host unreachable Host out of operation
TCP connection fail during transfer
Sender can re-queue mail
Give up after a period
Faulty destination address
User error
Target user changed address
Redirect if possible
Inform user if not

17. SMTP Protocol - Reliability
Used to transfer messages from sender to receiver over TCP connection
Attempts to provide reliable service
No guarantee to recover lost messages
No end to end acknowledgement to originator
Error indication delivery not guaranteed
Generally considered reliable

18. SMTP Receiver Accepts arriving message
Places in user mailbox or copies to outgoing queue for forwarding
Receiver must:
Verify local mail destinations
Deal with errors
Transmission
Lack of disk space
Sender responsible for message until receiver confirm complete transfer
Indicates mail has arrived at host, not user

19. SMTP Forwarding
Mostly direct transfer from sender host to receiver host
May go through intermediate machine via forwarding capability
Sender can specify route
Target user may have moved

20. Conversation SMTP limited to conversation between sender and receiver
Main function is to transfer messages
Rest of mail handling beyond scope of SMTP
May differ between systems

21. SMTP Mail Flow

22. SMTP System Overview
Commands and responses between sender and receiver
Initiative with sender
Establishes TCP connection
Sender sends commands to receiver
e.g. HELO<SP><domain><CRLF>
Each command generates exactly one reply
e.g. 250 requested mail action ok; completed

23. SMTP Replies Leading digit indicates category
Positive completion reply (2xx)
Positive intermediate reply (3xx)
Transient negative completion reply (4xx)
Permanent negative completion reply (5xx)

24. Operation Phases Connection setup Exchange of command-response pairs
Connection termination

25. Connection Setup Sender opens TCP connection with receiver
Once connected, receiver identifies itself
220 <domain> service ready
Sender identifies itself
HELO
Receiver accepts sender’s identification
250 OK
If mail service not available, step 2 above becomes:
421 service not available

26. Mail Transfer Sender may send one or more messages to receiver
MAIL command identifies originator
Gives reverse path to used for error reporting
Receiver returns 250 OK or appropriate fail/error message
One or more RCPT commands identifies recipients for the message
Separate reply for each recipient
DATA command transfers message text
End of message indicated by line containing just period (.)

27. Closing Connection Two steps Sender sends QUIT and waits for reply
Then initiate TCP close operation
Receiver initiates TCP close after sending reply to QUIT

28. Electronic Mailboxes and Addresses
users have an electronic mailbox into which mail is deposited
Usually associated with computer account; one user may have different electronic mailboxes
User accesses mail with a mail reader program
Electronic mailbox is identified by an e- mail address

29. E-mail message format Simple two-part format
Header includes delivery information
Body carries text of message
Header and body is separated by blank line

30. E-mail Headers Lines of text in format keyword: information
keyword identifies information; information can appear in any order
Essential information:
To: list of recipients
From: sender
Cc: list of copy recipients
Useful information
Reply-to: different address than From:
Received-by: for debugging
Frivolous information:
Favorite drink: lemonade
Phase-of-the-moon: gibbous

31. Data in E-mail Original Internet mail carried only 7-bit ASCII data
Couldn’t contain arbitrary binary values; e.g., executable program
Techniques for encoding binary data allowed transport of binary data
Uuencode: three 8-bit binary values as four ASCII characters (6 bits each)
Also carries file name and protection information
Incurs 33% overhead

32. Multipurpose Internet Mail Extension (MIME)
Extension to RFC822
SMTP can not transmit executables
Uuencode and other schemes are available
Not standardized
Can not transmit text including international characters (e.g. â, å, ä, è, é, ê, ë)
Need 8 bit ASCII
Servers may reject mail over certain size
Translation between ASCII and EBCDIC not standard
Some SMTP implementations do not adhere to standard
CRLF, truncate or wrap long lines, removal of white space, etc.

33. MIME
Multipart Internet Mail Extensions - Extends and automates encoding mechanisms
Allows inclusion of separate components – programs, pictures, audio clips – in a single mail message
Sending program identifies the components so receiving program can automatically extract and inform mail recipient
Header includes MIME-Version: 1.0 Content-Type: Multipart/Mixed; Boundary=Mime_separator
Separator line gives information about specific encoding Content-Type: text/plain

34. MIME (cont.)
MIME is extensible – sender and receiver agree on encoding scheme
MIME is compatible with existing mail systems
Everything encoded as ASCII
Headers and separators ignored by non-MIME mail systems
MIME encapsulates binary data in ASCII mail envelope

35. MIME

36. Overview of MIME Five new message header fields MIME version
Content type
Content transfer encoding
Content Id
Content Description

37. Content Types Text body Multipart Message Image Video Audio
Mixed, Parallel, Alternative, Digest
Message
RFC 822, Partial, External-body
Image
jpeg, gif
Video
mpeg
Audio
Basic
Application
Postscript
octet stream

38. MIME Transfer Encodings
Reliable delivery across wide largest range of environments
Content transfer encoding field
Six values
Three (7bit, 8bit, binary) no encoding done
Provide info about nature of data
Quoted-printable
Data largely printable ASCII characters
Non-printing characters represented by hex code
Base64
Maps arbitrary binary input onto printable output
X-token
Named nonstandard encoding

39. Message format: multimedia extensions
MIME: multimedia mail extension, RFC 2045, 2056
additional lines in msg header declare MIME content type
From:
To:
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
......base64 encoded data
MIME version
method used
to encode data
multimedia data
type, subtype,
parameter declaration
Multipurpose Internet Mail Extension (MIME)
Content-Transfer-Encoding indicates the method to convert to ASCII code
Content-Type indicates the type of content
encoded data

40. Multipart Type Break into parts? Compare with HTTP?
From:
To:
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain
Dear Bob,
Please find a picture of a crepe.
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
......base64 encoded data
Break into parts?
Similar to HTTP that the mail message contains many objects. HTTP uses one request/reply round to send a message, but SMTP sends different objects within one message.
Compare with HTTP?
2: Application Layer

41. Printable Encoding of Binary Data into Radix-64 Format

42. Programs as mail recipients
Can arrange for box to be associated with a program rather than a user’s mail reader
Incoming mail automatically processed as input to program
Example – mailing list subscription administration
Can be used to implement client-server processing
Client request in incoming mail message
Server response in returned mail reply

43. Mailing Lists and Forwarders
addresses can be attached to programs as well as electronic mailboxes
Mail exploder or mail forwarder resends copies of message to addresses in mailing list
UNIX mail program sendmail provides mail aliases
Mailing list processor, e.g. listserv, can also interpret subscription management commands

44. Mail Gateways
Mailing list processing may take significant resources in large organizations
May be segregated to a dedicated server computer: mail gateway
Provides single mail destination point for all incoming mail
Can use MX records in DNS to cause all mail to be delivered to gateway

45. Mail gateways and forwarding
Users within an organization may want to read mail on local or departmental computer
Can arrange to have mail forwarded from mail gateway
Message now makes multiple hops for delivery
Hops may be recorded in header
Forwarded mail may use proprietary (non-SMTP) mail system

46. Mailbox Access Where should mailbox be located?
Users want to access mail from most commonly used computer
Cannot always use desktop computer as mail server
Not always running
Requires multitasking OS
Requires local disk storage
Can TELNET to remote computer with mail server
Can use WEB based interfaces to access mailbox

47. Mail Access Protocols
Instead of TELNET, use protocol that accesses mail on remote computer directly
Mail access protocol transfers mail from server to local PC
2 popular protocols:
POP3 (Post Office Protocol v3)
IMAP (Internet Mail Access Protocol)
Can’t use SMTP because getting messages is a pull operation & SMTP is a push protocol.

48. Post Office Protocol (POP)
TCP/IP suite includes Post Office Protocol (POP) for remote mailbox access
Computer with mailboxes runs POP server
User runs POP client on local computer
POP client can access and retrieve messages from mailbox
Requires authentication (password)
Local computer uses SMTP for outgoing mail

49. POP3 and SMTP

50. POP3 Stands for Post Office Protocol – Version 3 RFC in 1993
POP3 “is intended to permit a workstation to dynamically access a maildrop on a server host in a useful fashion. Usually, this means that the POP3 protocol is used to allow a workstation to retrieve mail that the server is holding for it.“

51. POP3 Doesn’t provide extensive mail operations on the server
Normally mail is downloaded, then deleted
Deletion doesn’t really occur until QUIT
Uses port 110

52. POP3
Three phases
Authorization
Transaction
Update
During a POP3 session, the server maintains state information
POP3 downloads mail messages to the local machine – not good for “nomadic user”

53. POP3 protocol authorization phase C: list transaction phase, client:
S: +OK POP3 server ready
C: user bob
S: +OK
C: pass hungry
S: +OK user successfully logged on
authorization phase
client commands:
user: declare username
pass: password
server responses
+OK
-ERR
transaction phase, client:
list: list message numbers
retr: retrieve message by number
dele: delete
quit
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S: <message 1 contents>
C: dele 1
C: retr 2
C: dele 2
C: quit
S: +OK POP3 server signing off

54. Dialup Access and POP POP is useful for dialup connection
User’s computer not always connected
Can download all mail at once and read off- line
Can compose mail off-line and mail in one connection

55. Minimal POP3 Commands USER name – user’s name
PASS string – user’s password
QUIT – quits, and updates the mailbox
STAT – status
LIST [msg] – lists the mail messages (#’s only)
RETR msg – retrieves a mail message
DELE msg – deletes a message
NOOP – No operation
RSET – resets the mail to its original state

56. IMAP Internet Mail Access Protocol
Allows users to manipulate remote mailboxes
Must maintain info about user (e.g., folder hierarchy) even after the user quits
IMAP server is always in 1 of 4 states:
Nonauthenticated
Authenticated
Selected
Logout

57. POP3 (more) and IMAP More about POP3 IMAP
Previous example uses “download and delete” mode.
Bob cannot re-read if he changes client
“Download-and-keep”: copies of messages on different clients
POP3 is stateless across sessions
IMAP
Keep all messages in one place: the server
Allows user to organize messages in folders
IMAP keeps user state across sessions:
names of folders and mappings between message IDs and folder name

58. Security SMTP, POP3 and IMAP are plain text protocols
Secured version available to protect the user
SMTPS
POP3s
Need to be configured both on server and client

59. Stunnel Name of an application/service What does it do?
Even if the mail server application does not implement secure version
What does it do?
gathers the encrypted message from client
Decrypts the data
Sends plain text data using a local connection to insecure port
Server application considers it a normal client

60. STunneling Client Server Send decrypted data to port 25 of the server
(on server)
Listening to port 995
Encrypted data
(Command/mail body)
Client
Server

61. Web interface Nowadays there are many web interface user agents
They allow accessing messages from any client with only a web browser
However the interface is not as sophisticated as special mail agents
But it is very useful if you need to check your s from different computers

62. Summary for
Electronic mail allows quick, asynchronous communication across entire Internet
Can attach addresses to programs for processing
Mailing lists
Other client-server applications
Simple Mail Transfer Protocol (SMTP) is Internet standard for mail delivery
Mail gateways
Provide uniform user addressing outside organizations
Translate from Internet mail (e.g. SMTP) to proprietary systems
Post Office Protocol (POP) and Internet Mail Access Protocol (IMAP) allow remote access to electronic mailboxes.