The "Mail Days to Sync" setting defaults to 2 weeks, and emails older than that will be removed from the device.  To change this behavior:

For iOS 12/13 devices go to Settings>Passwords & Accounts. For iOS 14 devices, go to Settings>Mail>Accounts.
Select the Partners(OAuth) account.
Select "Mail Days to Sync".
Set to the desired time length.

Go to KB0038275 in the IS Service Desk

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