Office 365 offers already a lot of build-in Reports for the various Services within Office 365. You can find those Reports in the Admin Dashboard and the Menuitem Reports. Office 365 Administrators can look at those Reports. Please be aware that those reports are visible if you have at least one Exchange Online license within your account (Thanks Erwin B. from MS Support for helping me figure this out).
The provided reports include information about active and inactive mailboxes, mailbox usage, new and deleted mailboxes, deployed OneDrive for Business (SkyDrive pro) Reports. You also see active Lync Users and how much storage is left for your Teamsites. Every report is available as table or as graphic.
But there is more within Office 365: you can make your own reports about Mailtraffice with Excel. Use the Mail Protection Reports! Download the corresponding version of your installed Excel Version (in most cases this will be the 32bit version). Install the reports and open the Shortcut on your Desktop.
Click on Query and fill in username and password of an Office 365 Administrator. In the next step you can choose the itnervall: custom, 14 days, 30 days, last 24 hours and more. Rund the report, all data is queried with OData.
The results are really great and informative:
You can find here the hitlist of e-mail senders and recipients, Spam E-Mails, DLP Reporting and much more.
Have fun analyzing your Mailtraffic!