New Post: Error : Cannot open your default e-mail folders. the file ..outlook.pst is in use and cannot be accessed.

Unless you're calling into Simple MAPI, MapiLogOff isn't a MAPI function, so I assume it's something you wrote. Further, I assume in there you're calling IMAPISession::Logoff (http://msdn.microsoft.com/en-us/library/office/cc842296(v=office.15).aspx). I've found it best not to call that function. Try removing it (but otherwise releasing everything normally) and see what happens.