Check to see if there is a cookie called "email". It might be getting set in some other page on your site that has nothing to do with this mortgage application.
Of course, that wouldn't explain why the value isn't in $POST. If it's not in $POST, it could be that the mortgage application is including a Javascript that alters the contents of that variable when the form is submitted.
As a very basic test, just make a simple form (outside of the mortgage app) and a simple PHP script to print_r the $_POST variable. This will confirm whether the mortgage app is interfering with the post.
Then, try posting to the mortgage app from an HTML form you built yourself.
Then trying using the mortgage's Form to post to your own simple print_r script.
By isolating each part, you should be able to get a better idea of where "email" is losing its value.