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General architecture of an e-mail system:

Ø  There are three main components: user agent, message transfer agent, and message access agent.

a)   User Agent:

Ø  The first component of an electronic mail system is the user agent (UA).

Ø   It provides service to the user to make the process of sending and receiving a message easier.

Ø  A user agent is a software package (program) that composes, reads, replies to, and forwards
messages.

Ø  It also handles mailboxes.

b)   Message Transfer Agent (MTA):

Ø  The actual mail transfer is done through message transfer agents.
Ø  To send mail, a system must have the client MTA, and to receive mail, a system must have a server MTA. Ø  The formal protocol that defines the MTA client and server in the Internet is called Mail Transfer Protocol (SMTP). the Simple
Ø  As we said before, two pairs of MTA client/server programs are used in the most common situation.
Ø  SMTP is used two times, between the sender and the sender's mail server and between the two mail servers.
Ø  Another protocol is needed between the mail server and the receiver.

SMTP:

Ø  SMTP stands for Simple Mail Transfer Protocol. Ø  It was first proposed in 1982.
Ø  It is a standard protocol used for sending e-mail efficiently and reliably over the internet.
Ø  Key Points:

·      SMTP is application level protocol.

·      SMTP is connection oriented protocol.

·      SMTP is text based protocol.

·      It handles exchange of messages between e-mail servers over TCP/IP network.

·      Apart from transferring e-mail, SMPT also provides notification regarding incoming mail.

·      When you send e-mail, your e-mail client sends it to your e-mail server which further contacts the recipient mail server using SMTP client.

·      These SMTP commands specify the sender’s and receiver’s e-mail address, along with the message to be send.

·      The exchange of commands between servers is carried out without intervention of any user.

·      In case, message cannot be delivered, an error report is sent to the sender which makes SMTP a reliable protocol.

c)   Message Access Agent:
Ø  The first and the second stages of mail delivery use SMTP.
Ø  However, SMTP is not involved in the third stage because SMTP is a push protocol; it pushes the message from the client to the server.
Ø  In other words, the direction of the bulk data (messages) is from the client to the server.
Ø   On the other hand, the third stage needs a pull protocol; the client must pull messages from the server. The direction of the bulk data is from the server to the client. The third stage uses a message access agent.
Ø  Currently two message access protocols are available: Post Office Protocol version 3 (POP3) and Internet Mail Access Protocol, version 4 (IMAP4).