You need to have mailbox auditing enabled before anything you read in this post will work.
So now that you have auditing enabled, how do you find out who is accessing other people’s mailboxes?
The new version of Office 365 includes a mailbox access by non-owners report for exactly this purpose. These can be found in the Reports section of your Office 365 administrator portal, under Auditing section. You will need to be signed in as an Administrator or other user that has the Audit Logs role assigned.
Audit Reports in Office 365
Clicking on this Audit Report opens a new window.
Mailbox Access Audit Search
Here you can define the parameters which will
limit the scope of your search results. You can edit things such as:
- Date/Time of user access
- Which mailboxes have been accessed
- Who accessed the mailboxes – administrators, delegates etc
