Just try this code:
app = new Microsoft.Office.Interop.Outlook.Application();
Microsoft.Office.Interop.Outlook.NameSpace ns = app.GetNamespace(“MAPI”);
f = ns.GetDefaultFolder(OlDefaultFolders.olFolderInbox);
Thread.Sleep(5000); // a bit of startup grace time.
if outlook was open it uses it, if not its opened it. Of course, if your outlook requires you to login, your code wont allow for that. Some systems make it difficult for you to auto login. I hope it will assist you in resolving issues.
Thanks & regards