This quickly lead to software which could hold messages while people were not signed on. This software also adopted the metaphor of the postal system, and e-mail was born. It wasn't long before there were address books, folders for filing mail messages, and ways of sending single messages to many people at the same time. The important point is that users could exchange mail with others who had access to the same machine. It also had the potential to save paper, since everyone could read and respond to their mail on the screen.
Today there are many variations available and it has become quite confusing.
There are good packages available as freeware, as shareware, and as purchaseable software, which can be cheap or quite expensive. Lotus Notes is a good example of the latter type. The problem which quickly presents itself is a lack of standards for how the different e-mail packages talk with each other. More correctly, the problem is their inability to communicate. Each e-mail package has its own file structure and individual protocols for handling communication.