The various components of an E-mail system are:

1. UA (user agent)

2. MTA (message transfer agent)

3. MAA (message access agent)

• User agent (UA) : The first component of an electronic mail system is the user agent (UA). It provides service to the user to make the process of sending and receiving a message easier. A UA is a software package (program) that composes reads, replies to and forwards message. It also handles local mailboxes on the user computers.

  There are two types of UA : Command driven and GUI based. Command driven UA belong to the early days of electronic mail. It normally accepts a one character command from the keyboard to perform its task. Modern UA are GUI based. They contain graphical user interface (GUI ) components that allow the user to interact with the software by using keyboard and mouse.

• Message transfer agent (MTA) : The actual mail transfer is done through the message transfer agent (MTA). To send a mail, a system must a system must have the client MTA and to receive mail a system must have a server. MTA. The formal protocol that defines the MTA client and server in the internet is called Simple Mail Transfer Protocol (SMTP). SMTP is used two times between the sender and the senders mail server and between the two mail servers. SMTP simply defines how commands and responses must be sent back and forth.

• Message access agent (MAA): The first and the second stages of mail delivery use SMTP. However SMTP is not involved in the third stage because SMTP is a push protocol. The third stage needs a pull protocol. The client must pull messages from the server. The third stage uses a message access agent (MAA) POP (Post Office Protocol) and IMAP (Internet Mail Access Protocol)