If routine maintenance does not fix the corrupted ACL, try the following:
1. Make a new copy of the database and deselect Access Control List in the "Specify What to Copy" section of the Copy Database dialog box. This will bypass the corrupt ACL and create a default ACL that you can then modify as necessary.
2. Replace the design of a mail database with a non-mail template and then do a replace back to the mail template, which has been known to correct corrupted ACLs.
3. From the Administration client -> Files tab, select a healthy ACL database where the ACL is similar to the one that needs to be replaced. Right-click and select ACL.Copy. Select the bad ACL database then right-click and select ACL.Paste. This is an easy way to reset the ACL.
If the steps detailed above do not offer relief of the situation, it is possible that the ACL is irreparable in its current state or the scope of corruption exceeds the ACL itself. Refer to the "Database Corruption Troubleshooting Guide" for additional assistance.