Window management learning curve

Many people can handle using one window at a time, such as a mail client. But introduce another window and the learning curve goes up significantly. Now they must learn how to switch between windows, and learn how the two interact with each other. I think the curve then flattens out until you hit about 7 windows, the average number of maximum items we can grok at one time (7 menu items, 7 digit telephone number, 7 fields on a form). After about 7, aperson has to not only know how to navigate between them, but also find the right one easily.

My point is - adding contacts to the left pane of a mail client is a good way to simplify the interaction of sending mail. I'm not saying there should not be a contacts window, just that mail should have the option of showing them.