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Microsoft released more open protocol specifications this week (http://www.microsoft.com/presspass/press/2008/apr08/04-08ProtocolPR.mspx). We’re talking 14,000 more pages of documentation involving Office client and server products (including SharePoint technologies and Office Communications Server) and Exchange. That brings the total protocol documentation under this program up to more than 44,000 pages.
If you do low-level work involving SharePoint, Office, Exchange, Outlook, OCS, etc., you should give this documentation a look. Very few corporate developers should be wading into these waters. This is the realm of ISVs ... as a friend of mine used to say, "Never build anything you don't intend to sell." That concept certainly applies when it comes to understanding interop at this low level.
Here’s the intro site: http://www.microsoft.com/about/legal/intellectualproperty/protocols/default.mspx
Here are the guts: http://msdn2.microsoft.com/en-us/library/cc203350.aspx