Clearing your cache in Outlook 2016 for macOS
Sometimes emptying Outlook's cache will solve problems like slowness and synchronization issues.
- Make sure the computer is connected to the Internet and launch Outlook.
- In the left-hand navigation pane, ctrl-click or right-click the Exchange folder for which you want to empty the cache, then click Properties.
- On the General tab, click Empty Cache. After the folder is empty, Outlook will automatically download items from the Exchange server.