This is also considered the MRU list...
Quit Outlook.
On the Start menu, click Run.
In the Run dialog box, type regedit, and then click OK.
Locate the appropriate registry key for your version of Microsoft Windows:Microsoft Windows NT or Microsoft Windows 2000:
HKEY_CURRENT_USER\Software\Microsoft\WIndowsNT\CurrentVersion\Windows Messaging Subsystem\Profiles\user_profile\ 0a0d020000000000c000000000000046\101e0390
You should see the names of off all the folders you have been given access to. From here you can just highlight and delete whichever names you would like to get rid of.
Once you've done that you can go back into outlook to verify that the MRU history has been cleared.
Hope this works for you!
Jbwambone!