For ease of reference, I also append the order number to the email subject by adding the following to includes\modules\pages\returns\header_php.php


Code:
// adds order number to email subject
	$email_subject = EMAIL_SUBJECT . ' #' . $order_number;


// Send message
	zen_mail($name, $email_address, $email_subject, $email_text, $send_to_name, $send_to_email, $html_msg, 'returns');
    $html_msg['EMAIL_MESSAGE_HTML'] = $text_message;
    $html_msg['EMAIL_GREETING'] = '';
	$html_msg['EMAIL_WELCOME'] = '';
    $html_msg['CONTACT_US_OFFICE_FROM'] = OFFICE_FROM . ' ' . $name . '<br />' . OFFICE_EMAIL . '(' . $email_address . ')';
    $html_msg['EXTRA_INFO'] = '';
	  
	zen_mail($send_to_name, $send_to_email, $email_subject, $text_message, $name, $email_address, $html_msg, 'returns');
One could also append an RMA number to the email subject as per the following:

Code:
$email_subject = EMAIL_SUBJECT . ' #' . $order_number  . ' - ' . $rma_number;
which would appear as:

Subject: RMA #1234 - 1

Cheers