I am currently teaching an evening course in Ancient Greek. The benefits of e-mail in this context are numerous. It has proven for me to be a flexible, practical, time saving, and informative guide.
Most questions arise when students are doing homework on translation or composition exercises.
This eliminates a lot of frustration when students studying alone encounter the same types of dilemmas as their classmates. If I sense from their questions that several students have similar problems, I know what types of review drills or group exercises to use, or when I might be going too rapidly or too slowly. In sum, I get a better sense of both the overall and individual aspects of the course.