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.