If you are on version 7.0 an above make sure you run the Provisioning Task then enable the user.
If that does not work.
Access the SQL server
In the Enterprise Vault directory database Open the ExchangeMailboxEntry table. Click on the black SQL button at the top.
Run this query to fine the user where user01 is the Exchange alias or close to it. If the user is there then delete the line from the table and run the provisioning task again. Run the query again and the user will appear. Enable the user through the GUI.
SELECT *
FROM ExchangeMailboxEntry
WHERE (MbxAlias LIKE '%user01%')