Since it’s come up again - if you use new Outlook and Mail in Windows 11, it automatically syncs all your email and contacts into Microsoft’s cloud - even if you use a third party mail provider.

There’s no option to decline.

Pretty obviously, consider the risk.

https://support.microsoft.com/en-gb/office/sync-your-account-in-outlook-to-the-microsoft-cloud-985f9e19-d308-4e85-9d1d-0c6f32f8e981