Index: program/steps/mail/func.inc
===================================================================
--- program/steps/mail/func.inc	(revision 6cd00992a2c7de38e0ff7df152fba8ea89012dce)
+++ program/steps/mail/func.inc	(revision 2106ec7043a636f2f706b44a0418e8d3c72c4c04)
@@ -779,4 +779,6 @@
         continue;
       }
+    else if ($hkey == 'to' && preg_match('/^undisclosed-recipients[:;]*$/', $headers['to']))
+      continue;
     else if (in_array($hkey, array('from', 'to', 'cc', 'bcc')))
       $header_value = Q(rcmail_address_string($headers[$hkey], null, true, $attrib['addicon']), 'show');