Work offline in Outlook - Microsoft Support
https://support.microsoft.com/en-us/office/work-offline-in-outlook-f3a1251c-6dd5-4208-aef9-7c8c9522d633
WEBClick File > Account Settings > Account Settings. Select an Exchange or Microsoft 365 account, and then click Change. Under Offline Settings, move the slider to the desired amount of time, such as All, 12 months, 6 months, 3 months, or 1 month. Note: If you're using Outlook 2016, you can also select an amount of time as short as 3 days.
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