Microsoft Knowledge Base Article 894384 contains the following summary and introduction:
Microsoft has released the update for the Microsoft Office Outlook 2003 Junk E-mail Filter (KB894384).
This article describes the following information about the update.
This optional update provides the Junk E-mail Filter in Microsoft Office Outlook 2003 with a more current definition of the e-mail messages
that should be considered junk e-mail messages. This update was released in May 2005.