If you have ever had a user complain that their folders have gone missing the best way to restore them is:
Restore their mail file from backup.
Create a design copy of the database, save this with an NTF extension.
Then do a replace design on their live mail file selecting the NTF that you have just created.
Once complete the user will have their folders back. Remember to enable prohibit design refresh on their folders before changing back to the default mail template.