This page describes how to change the amount of mail Outlook keeps stored offline and manage your Cached exchange mode setting.
With an Exchange Online account, you decide how much mail is available when working offline with a slow or no network connection.

1. Click File > Account Settings > Account Settings.

2. Select an Exchange account, and then click Change.

3. Under Offline Settings, set the slider to your desired time frame which ranges from 3 days to All Email. Cached Exchange Mode, which downloads a copy of your messages, must be turned on. We recommend using "Cached Exchange mode" for Outlook on Windows, as it allows for a faster experience using Exchange, offline mailbox access, and a reduced load on the servers. 

Note: This should be change to "All" before archiving or exporting mail in order to ensure all mail is included in the archive file.