Use the OnAfterChange event on the Contact Name field. To quickly add an event, double click on the field, click on data validation button, there is an events dropdown, select OnAfterChange and press the edit button. In that event handler, go out to whatever datasource you have and get the e-mail address.
You cannot use the other two events listed in that dropdown because the DOM is locked and no changes can be committed.
Jason
http://www.k2distillery.com/