Electronic mail can be forwarded to third parties and is handled through user mailboxes like inbox and outbox. Early systems were command-based while newer ones have a graphical user interface. MIME allows non-ASCII content to be encoded and sent via email. POP3 and IMAP4 are protocols for accessing email on a server, with IMAP4 providing more folder organization and preview features than POP3. When sending email between different systems, user agents and mail transfer agents are needed on both the client and server sides.

1. Electronic Mail

7. Forwrding messages : forward the received message to third party with or
without extra information
Handling mailboxes : user agent create and maintain two mailboxes
inbox and outbox.
User agents are of 2 types
Command based : mail, pine and elm
GUI : netscape, outlook, eudora etc.

8. The main limitation of Email is that it can send messages only in 7 bit
ASCII format. Therefore it can not be used for languages that are not
supported by 7 bit ASCII characters ( French, german, Russian,
Chinese etc.)
The another limitation is that it can not be used to send binary files or
video or audio data.
MIME ( Multipurpose Internet Mail Extensions ) is a supplementary
protocol that allows non ASCII data to be sent through email.
The MIME transform non ASCII data at the sender side to ASCII data
and at the receiving side it is transformed back to the original data.

10. MIME Header

11. Data Types in MIME

12. Content-Transfer-Encoding

14. Mail Transfer Agent (MTA): SMTP
• The actual mail transfer is done through MTA

15. Commands Commands are sent from the client to the server. The fonnat of
a command is shown in Figure 26.18. It consists of a keyword followed by
zero or more arguments.
SMTP defines 14 commands. The first five are mandatory; every
implementation must support these five commands. The next three are
often used and highly recommended. The last six are seldom used.
Commands
HELO Sender's host name
MAIL FROM Sender of the message
RCPTTO Intended recipient of the message
DATA Body of the mail
QUIT
RSET
VRFY Name of recipient to be verified
NOOP
TURN
EXPN Mailing list to be expanded
HELP Command name
SEND FROM Intended recipient of the message
SMOLFROM Intended recipient of the message
SMALFROM Intended recipient of the message

16. Responses Responses are sent from the server to the client. A response is a
threedigit code that may be followed by additional textual information.
As the table shows, responses are divided into four categories. The leftmost
digit of the code (2, 3, 4, and 5) defines the category.

17. Code Description
Positive Completion Reply
211 System status or help reply
214 Help message
220 Service ready
221 Service closing transmission channel
250 Request command completed
251 User not local; the message will be forwarded
Positive Intermediate Reply
354 Start mail input
Transient Negative Completion Reply
421 Service not available
450 Mailbox not available
451 Command aborted: local error
452 Command aborted: insufficient storage
Permanent Negative Completion Reply
500 Syntax error; unrecognized command
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command temporarily not implemented
550 Command is not executed; mailbox unavailable
551 User not local
552 Requested action aborted; exceeded storage location
553 Requested action not taken; mailbox name not allowed

20. POP3 is deficient in several ways. It does not allow the user to organize her mail on the
server; the user cannot have different folders on the server. (Of course, the user can
create folders on her own computer.) In addition, POP3 does not allow the user to
partiall check the contents of the mail before downloading.
Internet Mail Access Protocol, version 4 (IMAP4)
IMAP4 is similar to POP3, but has more features with extra
functions
•A user can check the email header prior to downloading
•A user can search the contents of the email for a specific
string of characters prior to downloading
•A user can create, delete, or rename mailboxes on the mail
server
•A user can create a hierarchy of mailboxes in a folder for
email storage

21. POP3 and IMAP4

22. Electronic Mail (1,2)
• When the sender and the receiver of an e-mail are on the same system, we need
only two user agents.
• When the sender and the receiver of an e-mail are on different systems, we need
two UAs and a pair of MTAs (client and server).

23. Electronic Mail (3)
• When the sender is connected to the mail server via a LAN or a WAN, we need two
UAs and two pairs of MTAs (client and server).

24. Electronic Mail (4)
• When both sender and receiver are connected to the mail server via a LAN or a
WAN, we need two UAs, two pairs of MTAs and a pair of MAAs. This is the most
common situation today.