Note: this deletes any information not yet synced with the server
Rename the Outlook .ost file (if you get an error about "cannot open set of folders")
- Exit Outlook and Skype for Business.
- From the Start menu, choose Run.
- In the Run window, type this path in the Open box "%LOCALAPPDATA%\Microsoft\Outlook\"
- Click OK
- Find the .ost file and rename it (you may need to Show Hidden Files and Show Extensions to see the file)
- Restart the computer
- Restart Outlook
Outlook "New" (Mac)
- Choose Tools > Accounts.
- In the left pane, select the account you want to reset.
- On the bottom left, click the gear icon and select Reset Account from the list.
Outlook "Old" (Mac)
- In the navigation pane, Ctrl+click or right-click the Exchange folder for which you want to empty the cache, and then click Properties.
- On the General tab, click Empty Cache. After the folder is empty, Outlook automatically downloads the items from the Exchange server.