Please make sure emails.txt is same directory with notify.php in line 40, notify.php (if it's not, you need to edit to new file location)

$file = fopen('emails.txt', 'a+');

If everything is ok and email can't be saved. You can try to debug by inserting this code before line 40

if (!$file = fopen('emails.txt', 'a+')) {
echo "Cannot open file ($filename)";
exit;
}

if (fwrite($file, $email) === FALSE) {
echo "Cannot write to file ($filename)";
exit;
}