WordPress Exit Optin is developed to help you get your website visitors attention when they are about to leave your site. This can be used in any creative way you can think of – give promotion code with discounts, subscribe to newsletter, pool, etc.
This is plugin is very simple and lightweight. You only need to input the Popup Title, Description, and the form you are using to get the email. You can use Contact Form 7 or Gravity Forms, any for that has shortcode can do.