"Cached Exchange Mode" enables a better experience when you use your Outlook 365 account. In this 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.
Outlook 2016 for Windows
- Open Outlook 2016 on your Windows computer.
- Click on the File menu option.
- Under Account Information, click on the Account Settings button and select Account Settings... from the drop-down list box.
- On the E-mail Accounts dialog box, double-click on the email account name listed.
- On the Server Settings dialog box:
- Make sure Use Cached Exchange Mode is checked
- Drag the "Mail to keep offline" slider from left to right to increase the amount of historical email you would like stored on your computer. If you drag the slider all the way to the right, you will see all your historical email. (Warning: This might take a while.)
- Click on the Next > button.
- Click on the Finish button.
- Exit and then restart Outlook.
If you still feel that emails are missing, use the following link to log into your Office 365 account online and search there. Your online account should contain all historical email available.