Office 365, Email, Exchange, Change how much mail to keep offline
How do I change the amount of mail Outlook keeps stored offline?
This page describes how to change the amount of mail Outlook keeps stored offline.
With a Microsoft Exchange Server 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.
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.
Note: This should be change to "All" before archiving or exporting mail in order to ensure all mail is included in the archive file.