Microsoft Knowledge Base Article 320528 contains the following summary:

Many environments require you to make anonymous queries to Active Directory. For example, you may have to make anonymous queries to return e-mail addresses. You can configure Active Directory to allow these queries.

This article describes how to configure Active Directory to support anonymous queries even though allowing anonymous queries can weaken the security of Active Directory. Use caution when you apply permissions to Active Directory because a misconfiguration may allow non-authenticated users to query for secure information. As a general rule, only give the Anonymous Logon account the permissions that are required to perform the anonymous query.