Use Mail unless username and password are required for your smtp server. Most restrict by IP.
To debug, in the file mail_PHP.php... update the code where it says:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader);
and add code afterward:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader);
var_dump($mailObj);
die();
That may tell you more about why the email won't send.