This is a bug of Outlook 2003.
Please download Service Pack 2 for Office 2003:
http://office.microsoft.com/en-us/officeupdate/default.aspx
and set this registry key:
"Start/run/regedit" =">" click on "OK"
="==========================
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Office\11.\Outlook\Options\General]
"AddUserToServiceSecurity"="dword:00000001
=============================
This key has to be set per Windows user!
Related Knowledge Base Articles:
http://support.microsoft.com/?kbid=885868
This can apply only with Outlook 2003 and Public ShareFolder as Windows Service.