How to generate inbound mails report?
Perform the following steps to generate Inbound Mails Report:
Select Exchange Online Reports from the available vertical tabs. Then from left pane of application, select Distribution Group Activity.
On the selection of "Inbound Mails" from the list of available reports, a dialog is displayed as shown below:

On selecting the distribution group and date range, the resultant data are displayed in a window as shown below: