Microsoft has made the Exchange 2010 transport server role architecture diagrams available for download. The Hub Transport Role Architecture diagram can help you understand the different transport components involved in processing and routing messages, the different transport agents that act upon messages and the events on which they are triggered, and also visualize the mail flow. The Hub Transport Extensibility diagram can help you understand how different transport agents process a message in the Exchange 2010 transport pipeline. Download both diagrams by clicking here.