neowin[.]net/news/new-outlook-for-windows-wants-to-send-your-non-msa-data-to-its-servers-for-your-own-good/
Microsoft has been working on a new, unified Outlook app to replace all the different versions, e.g., Windows, Android etc. They're going to force it on everyone next year, replacing Windows current Mail & Outlook apps, even though it's feature incomplete. If I understand the Neowin article correctly [there's not much in the way of documentation] non-outlook[.]com emails will be stored in Microsoft's cloud.