Microsoft Exchange Server 2007 Protocol Documentation v1.04 is now available to download.
The Microsoft Exchange Server 2007 Protocol technical documentation set provides detailed technical specifications for Microsoft protocols and extensions to industry-standard or other published protocols that are implemented and used in Microsoft Exchange Server 2007 to interoperate or communicate natively with Microsoft Office client and other server products.
The documentation set includes a set of companion overview and reference documents that supplement the technical specifications with conceptual background, overviews of inter-protocol relationships and interactions, and technical reference information.
And don't forget to read the Microsoft Exchange Server 2007 Standards Documentation.
The Exchange Server Standards Documents provide information about Exchange 2007 support for industry messaging standards and Requests for Comments (RFCs) related to iCalendar, IMAP4, and POP3.