Here's the syntax of mail();
mail(string to, string subject, string message, string addition_headers);
As explained in the second reply it's easy to send email, as for additional headers, you can add stuff like From: and Reply-To: like this:
mail($to, $subject, $message, "From:thesubmitter@theirsite.com\r\nReplyto:thesubmittersson@somesite.com");
That way the user can send feedback with their address and have it automatically set as the return address.