diff --git a/include/class.mailfetch.php b/include/class.mailfetch.php
index 07a3aa27fc46db8fe6644f7926f29c11822404cf..283c1aee4d7adb591178f30668989caa37bd730e 100644
--- a/include/class.mailfetch.php
+++ b/include/class.mailfetch.php
@@ -504,7 +504,8 @@ class MailFetcher {
             }
         }
 
-        if (($thread = ThreadEntry::lookupByEmailHeaders($vars))
+        $seen = false;
+        if (($thread = ThreadEntry::lookupByEmailHeaders($vars, $seen))
                 && ($message = $thread->postEmail($vars))) {
             if (!$message instanceof ThreadEntry)
                 // Email has been processed previously