in the mail_PHP.php file, try changing:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader);
to:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader, "-f".$mailFrom);
if it is still not working, I would contact the host to see if they can look into the SMTP servers error logs to determine why the email is not being relayed.