Out of the box, Office 365 offers more than 30 excellent reports for its various services, covering all the important aspects, from mailbox to traffic statistics, from Lync to SharePoint.
These reports can be found under reports in the Admin Dashboard. As a side note, please be aware that Exchange Online reports are only visible if you have at least one Exchange Online license within your account.
However, what some admins are not aware of, is that they can generate further mail traffic reports using the Mail Protection Reports! Simply download the msi appropriate for your Office installation (rather than the operating system platform), install it and then open the shortcut on the Desktop.
Click on Query and fill in username and password of an Office 365 Administrator for your tenant. Next, you can chose an interval of time from which to collect data:
Click OK and the tool will start gathering information:
The results are really impressive:
Available are reports around mail traffic, spam, malware, rules, data loss prevention, and more, with some reports being very customizable!