Note: this deletes any information not yet synced with the server

Outlook (Windows)

Rename the Outlook .ost file (if you get an error about "cannot open set of folders")

1. Exit Outlook and Skype for Business.
2. From the Start menu, choose Run.
3. In the Run window, type this path in the Open box  "%LOCALAPPDATA%\Microsoft\Outlook\"
4. Click OK
5. Find the .ost file and rename it (you may need to Show Hidden Files and Show Extensions to see the file)
6. Restart the computer
7. Restart Outlook

Outlook "New" (Mac) 

1. Choose Tools > Accounts.
2. In the left pane, select the account you want to reset.
3. On the bottom left, click the gear icon and select Reset Account from the list.

Outlook "Old" (Mac) 

1. In the navigation pane, Ctrl+click or right-click the Exchange folder for which you want to empty the cache, and then click Properties.
2. On the General tab, click Empty Cache. After the folder is empty, Outlook automatically downloads the items from the Exchange server.