So I have edited mailer.php to include the following lines:
//check user-type valid email
if (! check_email($submitted_email)) {
redirect($return_url, 'The email address you have typed is not valid. Please try again.');
}
This will exit the mailer and give the user a warning that they have not typed a valid email.
Could this be included as a standard or at least as an option in future releases?