[Plugin: Contact Form 7] Cannot use form without email field

I want to create a simple form:

Mobile Number-

Amount- with 3 radio buttons

For simple top ups.

However as soon as i remove the email field from the default contact form it brings errors, i can remove every other field but this one. Obviously for a mobile top up, email address is not required

http://wordpress.org/extend/plugins/contact-form-7/