November 4, 2022
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.