Instead of that, I adjusted the code the following way. Then you from is dynamically generated from the plugin options.
Basically, it is a slight change to the mrt_all_from variable and then using you “-f address” trick.
$mrt_all_from = "-f". get_option( "mrt_sms_from" );
$body = $_POST['message'];
$subject = $_POST['subject'];
$to = $sendnum . $carsuf;
$headers = 'From: ' . $mrt_all_from . "\r\n" .
'Reply-To: ' . $mrt_all_from . "\r\n" .
'X-Mailer: PHP/';
if (mail($to, $subject, $body, $headers, $mrt_all_from)) {
