MS built Win10 to spy on you and build a business from data mining. Why would they not do that for email? (Or, more likely, change Outlook's ToS to allow it once you are hooked.)
You could say that of any email provider, but ProtonMail is under GDPR so it makes it a safer alternative.
Source: https://www.blog.google/products/gmail/g-suite-gains-tractio...