tsnor :
The data flow:
1. Your PC's Cortana agent sends an unencrypted copy of your email to microsoft
2. Microsoft servers use machine learning alogrithms against your email
3. Microsoft servers create a reminder and send it back to your PC
4. Microsoft does ** NOT ** use your email contents for any other purposes. Really. And there is ** NO ** way your privacy is compromised by having your emails on Microsofts servers in the clear. They will not collect information you, your friends and your family this way, nor is any government agency involved in examining the content of the emails.
Be careful what you joke about in email.
It's the principle of the thing. Data collection like this, for whatever purpose, should be opt-in, not opt-out. Furthermore, there should be a proper ability to opt-out, which right now, there isn't. You can't turn off the data collection, all you can do is make it invisible to the user.