2.1 End user’s point of view

From the end user’s point of view, email is quite simple. A user can check email at any time, and send email at any time. After a message is sent, it should be delivered relatively promptly to its recipient. That’s about it.

The end user’s point of view is deceptively simple. From the implementation point of view, there are two big problems:

• When a sender sends a message, what happens if the recipient is off line?
• When a user is ready to read messages, how does the client machine get the messages sent to this user when this user was off line?