Microsoft has released four new whitepapers detailing how Exchange 2007 is used internally. I am a big fan of these papers. I like to see how the maker designs, uses and maintains its own software. You always find some good tips and previously unknown tricks. Some of the best sessions I have ever been to at TechEd have been the "how we do this at Microsoft" ones.
Tip of the hat to You Had Me at EHLO