Does anybody know some common causes of queues backing up in the Messages waiting to be routed queue? The queues started to back up for about 30min to an hour but drained by itself. I turned up diag logging for transport but did not any relevant errors. This occurred on only one Exchange 2k server. I'm thinking it was some how performance related as these boxes are already crawling, or possibly a large message\corrupt in the queue which I forgot to check before it drained. Thanks.