WP MAIL

$mailer = WC()->mailer();
$rml_in_mail_subject = 'Order#' . $this_get_id . ' Status Changed;
$message_body="";
$to = "";
$subject = "";
$message = $mailer->wrap_message($rml_in_mail_subject, $message_body);
$mailer->send($to, $subject . $to, $message);