JSI Tip 9419. A software update is available to help prevent the enumeration of Exchange Server 2003 e-mail addresses.

Microsoft Knowledge Base Article 899492 contains the following introduction:

This article discusses a software update that you can install to help prevent the enumeration of e-mail addresses in your Microsoft Exchange Server organization. You can install this update if you run Microsoft Exchange Server 2003 on a Microsoft Windows Server 2003-based computer.