I believe there's a registry hack for versions of outlook earlier than 2003, but I haven't looked for it.
Outlook 2003 apparently has the ability to minimise to the try built into it...
Put a shortcut to it in the startup group. Right click it and goto the properties. Change the run property to minimized. Then...
Right click the tray icon and select hide when minimized. Now when you run it you'll only see the tray icon.
Well, I tried putting the shortcut in the startup group and it started all right but it was slightly smaller than maximized. I did change the properties to run minimized and checked the hide when minimized button.
Any other ideas before I try trayiconizer?