Event ID:9551
Source:MSExchangeIS Mailbox Store
Component:Microsoft Exchange Information Store
Message:An error occurred while upgrading the ACL on folder <folder name> located on database "<name>".
The Information Store was unable to convert the security for <distinguished name> into a Windows 2000 Security Identifier.
It is possible that this is caused by latency in the Active Directory Service, if so, wait until the userrecord is replicated to the Active Directory and attempt to access the folder (it will be upgraded in place).If the specified object does NOT get replicated to the Active Directory, use the Microsoft Exchange System Manageror the Exchange Client to update the ACL on the folder manually.
The access rights in the ACE for this DN were <number>.

This error is related to the appearance of zombie users. Zombie users are users that appear in an Access Control List (ACL) but not in Active Directory.

During an upgrade from an Exchange 5.5 server to an Exchange 2000 server (or install of Exchange 2000 or 2003 into a 5.5 site), a configuration connection agreement (CA) is automatically created. When Active Directory Connector (ADC) replication begins, a message displays because users are not in Active Directory yet, and the server enters a partial migration state.

An administrator or a folder owner must configure a user agreement so that Exchange can populate Active Directory with the users from the Exchange 5.5 directory service. Users that are not in the Exchange 5.5 directory service, but belong to an access control list (ACL), become zombie users in an Exchange 2000 organization. These users are typically obsolete accounts, not valid users.

User Action

To correct zombie users in an Exchange organization, use System Manager to access each folder specified in the message. In the folder Properties, click the Permissions tab, and then click Client Permission. Manually remove invalid user accounts.

In a mixed mode organization, run the DS/IS Consistency Checker on an Exchange 5.5 server to remove invalid users.

Currently there are no Microsoft Knowledge Base articles available for this specific error or event message. For information about other support options you can use to find answers online, see