How to generate Non-Owner Mailbox Access?
Non-Owner Mailbox Access shows the list of all users who have permissions to the other user's mailboxes. This report shows information such as Mailbox Name, Mailbox E-mail Address, Identity, User Name, User E-mail Address and Access Rights.
Perform the following steps to generate Non-Owner Mailbox Access:
Select Office 365 Manager from the available vertical tabs. Then from left pane of the application, select Security Reports and then click on Non-Owner Mailbox Access.
On selecting "Non-Owner Mailbox Access" Report, a window is displayed as shown below:

Once the data collection process is complete, the report data will be displayed in a report window as shown below: