Enable Cached Exchange Mode
In Outlook 2013/2016/2019:
- In Outlook, go to the File > Account Settings, then click on Account Settings again in the dropdown.
- In the E-mail tab, select your Exchange account and click on Change.
- In the Exchange Server Settings window, check Use Cached Exchange Mode and click on Next.
- You should be back at the Email Accounts window. Click on Finish.
You should see a notification window that advises you to restart Outlook in order for the change to take effect:
When you re-launch Outlook, it will begin to make a copy of your Mailbox content to your computer. This will take some time. Not all items will be immediately available. If you have a large mailbox or a slow Internet connection, it can take an hour or more.