DLP

DLP is a system that allows you to create sophisticated policies to prevent the loss of information in your organization. At its simplest, DLP can be configured to prevent accidental or purposeful deletion of emails or files that match certain business rules. For example, we may create a policy that places a hold on any email containing Top Secret in the subject or body.

DLP is a sophisticated feature of Office 365, and it could easily fill its own book. If you're interested in implementing this kind of functionality, it's probably best to seek the help of a Microsoft partner who specializes in DLP and cloud security in general. Such partners will have access to industry-specific and ready-made templates, which will greatly accelerate your efforts.

Data loss prevention in Exchange 2016: 
https://technet.microsoft.com/en-us/library/jj150527(v=exchg.160).aspx.