mail routing configuration allows every server to be selected (Fixes #5068)

interface/web/mail/form/mail_transport.tform.php

@@ -68,7 +68,7 @@ $form["tabs"]['transport'] = array (
 			'formtype' => 'SELECT',
 			'default' => '',
 			'datasource' => array (  'type' => 'SQL',
-				'querystring' => 'SELECT server_id,server_name FROM server WHERE mirror_server_id = 0 AND {AUTHSQL} ORDER BY server_name',
+				'querystring' => 'SELECT server_id,server_name FROM server WHERE mirror_server_id = 0 AND mail_server = 1 AND {AUTHSQL} ORDER BY server_name',
 				'keyfield'=> 'server_id',
 				'valuefield'=> 'server_name'
 			),