Hmm.
Couple of people have raised calls regarding this.
Surprisingly (well, not), the documentation from Microsoft is rather bizarrely.. well, Microsoftised.
However, it is decent background reading anyway, so I will paste here the required links (for Exchange 2007 mind), and possibly later post with a “shortened” version for normal people to understand.
Certificate Uses in Exchange Server 2007
TLS Functionality and Related Terminology in Exchange 2007
How to Configure Mutual TLS for Domain Security.
Fantastic (eugh) reading, so get ready to be Microsoftised and enjoy it 
