I needed to look up a piece of trivia on the Exchange routing engine for the cookbook, and after a little Googling I found this gem: the Exchange Server 2003 Transport and Routing Guide. I'm not sure how I missed it before, but it's quite comprehensive. Recommended reading if you want a better understanding of how the transport core works. In particular, its description of how the various connection filtering pieces work together is almost as good as what I wrote in Chapter 8 :)