Fixing the Outlook.ost Error - Cannot Dismiss Old Calendar Items

This can happen if your internet connection is dropped when connected to an Exchange server while in the Exchanged Mode. If using the scanost.exe and scanpst.exe do not resolve the issue...here is how you can work around it and get back on track:

1. Open Outlook.
2. Go to Tools > Account Settings.
3. In the email tab double-click on the type that says Exchange (this is your primary account).
4. Clear the Use Cached Exchange Mode.
5. Click Next.
6. Click OK on the dialog that comes up to tell to you restart Outlook for the changes to apply.
7. Click Finish.
8. Close Outlook.
9. Open Windows Explorer and browse to :\Documents and Settings\\Local Settings\Application Data\Microsoft\Outlook.
10. If you cannot see the folder...you will have to enable viewing of Hidden Folders using Tools > Folder Options and on the View tab place a checkmark in the "Show hidden files and folders" and click OK.
11. Rename the Outlook.ost to outlook.ost_old.
12. If you have trouble renaming it, close Communicator or any tools that may be using Outlook (look in the Task Manager).
13. Start Outlook.
14. Go to Tools > Account Settings.
15. In the email tab double-click on the type that says Exchange (this is your primary account).
16. Put a checkmark the Use Cached Exchange Mode.
17. Click Next.
18. Click OK on the dialog that comes up to tell to you restart Outlook for the changes to apply.
19. Click Finish.
20. Close Outlook.
21. Open Outlook.

Tested this on Windows XP SP 3, using Outlook 2007.