http://blogs.msdn.com/stephen_griffin/archive/2009/05/22/the-fifth-mapi-multithreading-rule.aspxWe had an issue recently where DDE broadcasts were being blocked on a system. The customer noticed that the problem happened after they called MAPIInitialize on their worker thread. Our investigation gives us a chance to add a 5th rule to the MAPI Multithreadingen-USCommunityServer 2.1 SP1 (Build: 61025.2)