How to reset an OST file

To reset .ost files

1. Start Outlook in Offline mode, and salvage desired items to an Outlook data file (.pst file).
2. Start Outlook again, and select Connect to go online with the new Exchange database.
3. On the Tools menu, click E-mail Accounts.
4. Click View or change existing e-mail accounts, and then click Next.
5. Click the Microsoft Exchange Server account, and then click Change.
6. Click More Settings, and then click Advanced.
7. Clear the Use Cached Exchange Mode check box, and then click Apply.
8. Click Offline Folder File Settings, and then click Disable Offline Use. A prompt appears: “Exchange is
   currently in recovery mode. If you make changes to your Offline Folder Files settings, your existing .ost file
   will be removed from your profile and you will not be able to use the file. Do you want to continue?” Click
   Yes. Click OK, Next, and Finish until you have exited the E-mail Accounts wizard.
9. Close Outlook.
10. Start Outlook again, and then click Connect to go online with the new Exchange database.
11. On the Tools menu, click E-mail Accounts.
12. Click View or change existing e-mail accounts, and then click Next.
13. Click the Microsoft Exchange Server account, and then click Change.
14. Click More Settings, and then click Advanced.
15. Select the Use Cached Exchange Mode check box.
16. Click Offline Folder File Settings, and then change the name of the .ost file.
    For example, if the file name is outlook.ost, name it outlook1.ost. You can also change the file path if you
    want to.
17. Click OK, and a prompt appears: “The file filename could not be found. Would you like to create it?” Click
    Yes, and then click OK, Next, and Finish to exit the E-mail Accounts wizard.
18. Exit, and then restart Outlook.