Limit Form Submissions by Email Address?

I am trying to have a contest on my Wordpress powered site. I am using cforms plugin, but I can not figure out how to limit the entries (form submissions) to one per email address. Can someone help me out?

I’m not familiar with cforms plugin but it looks like a contact form plugin, and limiting one submission per email is probably not what most users of contact forms have in mind.

How about using an autoresponder instead? That way not only you’ll be able to limit entries per email but it will also be much easier to manage and contact your contestants later on. Not to mention the obvious that you’d be building a list.