Tell HN: Outlook App copies external email data to Microsoft Cloud
support.microsoft.com
support.microsoft.com
According to the linked support article, there are no options to disable this "feature" aside from removing the account. I used a new account to test it out, and was unable to find out where the data was synced to and there's no way to verify it was deleted completely after I removed the account.
Regardless whether the data is deleted or not, I find troubling that this synchronization with the Microsoft cloud is forced on the user (if I get this right you cannot add an e-mail account to outlook without opting in for this synchronization).
There is no opt-out, unfortunately, either during the add process or after the fact.