Click File > Account Settings > Account Settings.
Click the Exchange or Office 365 account, and then click Change.
Under Offline Settings, check Use Cached Exchange Mode.
(If you're an Office 365 subscriber with semi-annual updates, under Offline Settings, check Use Cached Exchange Mode to download email to an Outlook data file.
4. Exit, and then restart Outlook.
If you want to turn off Cached Exchange Mode, repeat the steps, but clear Use Cached Exchange Mode.