It is Ok to have fear when your data leaves the company.
It is OK to ask yourself some legit question when using Office 365 as your Cloud provider for Exchange and SharePoint.
Fear not my friend!! Office 365 has you back covered..
Here is a cool video from Microsoft Ignite that held in Chicago in May.
And here is another 5 minutes video: https://www.microsofttrends.com/2014/05/26/technical-details-on-office-365-fort-knox-encrypted-storage/
Office 365 is not made for every business. Every business has its own policy and compliance. But at least, you know that your data is pretty safe up there.