Markus Klein has written a new article over at www.msexchange.org titled Troubleshooting and Monitoring Exchange Server 2003 Message Flow.
Nowadays, with the great success of Exchange Server 2003, you have to prepare yourself on what action to take should message flow related problems occur. To ensure that you, as an administrator, become aware and recognize any problems before your users have reported them, you will need to make use of monitoring. This article will describe how to monitor your server queues and how to react if there are any problems. [MS Exchange Blog]