Clear form fields after submission

this is my first question!

I'm working on a simple email spoofer, but the problem is, when you click the submit button, the form fields save the originally entered values, and that means you can spam someone with tons of the same email easily by either pressing the submit button a lot or refreshing the page.

I would like some assistance.

Index.php - http://ift.tt/1HoXMl1

Mail.php - http://ift.tt/1B5BSvX