Index: frontend/php/include/sendmail.php
===================================================================
--- frontend/php/include/sendmail.php	(revision 6514)
+++ frontend/php/include/sendmail.php	(working copy)
@@ -138,9 +138,13 @@
   $more_headers .= "Message-Id: <".$msg_id.">\n";
   # Add refs
   if ($savane_tracker && $savane_item_id)
-    {      
-      $more_headers .= "References: ".trackers_get_msgid($savane_tracker, $savane_item_id)."\n";
-      $more_headers .= "In-Reply-To: ".trackers_get_msgid($savane_tracker, $savane_item_id, true)."\n";
+    {
+      $refs = trackers_get_msgid($savane_tracker, $savane_item_id);
+      if ($refs)
+        {
+          $more_headers .= "References: ".$refs."\n";
+          $more_headers .= "In-Reply-To: ".$refs."\n";
+        }
     }
 
   # Add a signature for the server (not if delayed, because it will be added