In EWS Managed API is it easy to create an appointment for a specific user:
ExchangeService service = new ExchangeService();
service.Credentials = new NetworkCre
Folder inboxFolder = Folder.Bind(service, new FolderId(WellKnownFolderName.Inbox, "user1@example.com"));
Will work too. Then pass inboxFolder.id to the Appointment.Save call. The updates and deletes don't need this. The best answer is to use impersonate, but this requires it to be enabled by the server admins. If you don't wield such power, this method will let you do what you need. Note: the user running your application must have permissions on the target account or this will fail (as it should).
Found here: http://msdn.microsoft.com/en-us/library/gg274408(v=EXCHG.80).aspx