Restore seen but previously rejected feature

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