Force people??? You could twist their arm up their back, threaten violence etc!
I think you are talking about required fields. Pretty much any form-to-email script will do that.
Doesn't surprise me at all that people "forget" to enter their email address. In many cases a visitor might not want to enter their email address. I never require the email address on a straight contact form.
You might get someone wanting to give you quick valuable feedback but who don't want to enter their email address. I for one would not deter such people.
Generally speaking, I would be reluctant to use a contact form with a lot of required fields on it.