E-mail

One of the most common network applications is electronic mail or e-mail.
E-mail is fundamentally different from paper-based communication.
Since the turnaround time is fast email is more conversational than traditional paper-based media.E-mail messaging systems use the 'store and forward' principle to send and receive messages. The server acts as a central repository for messages i.e. if a user who is not currently connected to the network receives a message it is stored at the server until the user connects and downloads the messages.
Components of an E-mail system The components of an e-mail system are as follows:
The User Agent/Email Client
This performs tasks such as composing mail maintaining address books notifying users when an email arrives and sorting mail after it arrives in the Inbox.
The Message Store/Post Office
This holds the mailbox for each user and ensures that incoming messages are passed to the correct recipient.
The Message Transfer Agent (MTA)
This is responsible for forwarding messages when they are destined for a recipient located on another Post office.
The Access Unit/Gateway
This is used to send messages mail systems that use software and standards different from the one you are using e.g. LAN based messaging systems that communicate with Internet mail servers.
gateway must be able to physically route messages to their intended destination message store and must also translate the message content and reply address to a format understood by the destination system.
There are various levels of sophistication in a messaging system.
Some provide advanced features such as delivery receipt and encryption of messages.
Some of the widely used LAN based messaging systems are:
Microsoft Exchange
Lotus CC Mail and Notes
GroupWise