Originally posted by: stingygrrl
a google search netted me this registry hack:
Get hacking
Remember, this only works if you have Outlook XP (not Outlook Express). If you are missing the registry key below, this tip will not work for you. Always remember to backup your registry before you mess with it.
Open Regedit.
Go to HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Preferences
Add a new DWORD value and name it MinToTray
Set it equal to 1.
Restart Outlook, it should now minimize to the tray.
source:
copied the hack from here