I realized this when Microsoft started reading the emails and attachments in my office 365 account, and having Cortana "helpfully" suggest various to-do items from them. The degree to which this seems to be the case has me convinced that the only way I will actually have any control of my data is if it's on a hard drive in my physical possession. And frankly I don't have time to maintain my own IT infrastructure.
If you don't like that o365 is scanning your emails for todos and potential calendar events but it would be a non-issue on your Mail-in-a-Box then you have a problem with MS not the scanning.