Explanation/Reference:
Use mail protection reports in Office 365 to view data about malware, spam, and rule detections.
If you're an Exchange Online or Exchange Online Protection (EOP) admin, there's a good chance you'd like to monitor how much spam and malware is being detected, or how often your transport rules are being matched. With the interactive mail protection reports in the Office 365 admin center, you can quickly get a visual report of summary data, and drill-down into details about individual messages, for as far back as 90 days.