--- rcube_storage.php	2012-11-29 00:20:04.000000000 +0100
+++ rcube_storage.php.delivered_envelope	2012-12-05 22:28:43.000000000 +0100
@@ -65,6 +65,7 @@
         'MAIL-REPLY-TO',
         'RETURN-PATH',
         'DELIVERED-TO',
+        'ENVELOPE-TO',
     );
 
     const UNKNOWN       = 0;