Microsoft Knowledge Base Article 919089 contains the following summary:

This article describes a problem that may occur when the Directory Service Access (DSAccess) component cannot communicate with the Active Directory directory service. To resolve this problem, you must verify the following:

•    The default domain policy or the default domain controllers policy is not blocked by a "No override" configuration on an organizational unit object or on the domain object.
•    The Manage Auditing Security Privilege user right in the default domain controllers policy is applied to the Exchange Enterprise Servers group.
•    The computer account of the affected Exchange server is included in the Exchange Domain Servers group.