Electronic mail systems allow users to compose and send messages to other users. They utilize user agents to create and manage mailboxes, as well as mail transfer agents (MTAs) like SMTP to transmit messages between systems. MIME extensions allow non-ASCII content like images and files to be included in email messages by encoding them. IMAP and POP3 are common protocols for accessing email on a server, with IMAP providing more features for organizing and searching mail than POP3.

1. Electronic Mail

7. Forwarding 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 are sent from the client to the server. The format 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 are sent from the server to the client. A response is a three digit 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
partially 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.