In cached exchange mode, a copy of your mailbox is saved on your computer. This copy provides quick access to your data, and it is frequently updated with the server that runs Microsoft Exchange.
Cached Exchange Mode is the default when you add an Exchange or Office 365 account.
We recommend always using Cached Exchange Mode with an Exchange or Office 365 account.
Click File > Account Settings > Account Settings.
Click the Exchange or Office 365, 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.