If the private store used to log onto an Exchange server has its default public store (or default public folder server in Exchange 5.5) set to a server other than the target (that is, the server which is hosting the log on store), it can cause the public folder enumeration to fail.
Therefore, in order for MessageStats to be able to gather from the these four servers, they must go into each of the four servers and change the default public store for each of the four servers to be that of a public store on the same server. In addition, if there have deleted the System Mailboxes, this will also cause problems and will need to be corrected by using the following Microsoft Knowledge Base Article.
Also confirm that there is a SystemMailbox for every mail store in your organization.
To verify the existence of a SystemMailbox, launch ADSI Edit, which is available from the Windows 2000 Server CD and is included in the Support Tools.
Expand Domain NC --> DC=<domainname> --> CN=Microsoft Exchange System Object.
Please ensure a SystemMailbox is configured for every mail store in your organization.
In System Manager, verify the presence of a SystemMailbox. Launch System Manager, expand "Administrative Group" --> "First Administrative Group" --> "Servers" --> "<servername>" --> "First Storage Group" --> "Mailbox Store" --> Mailboxes. Verify the existence of a SystemMailbox mailbox. Please check subsequent Mailbox stores.
Please refer to the following Microsoft Knowledgebase article to recreate the System Mailbox: