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.