This will help you determine if mailbox access observed with Admin/Delegate Logontype. The logon type indicates mailbox accessed from non-owner user. Exchange allows Admin and delegate permissions to access other user's inbox.
Elevation of Priviledge
MITRE ATT&CK Tactics
Recurrent and approved O365 operational activities within your organization
1. Review generated events via Azure Sentinel console.
2. If delegated access is provided to given users, you can whitelist those and investigate the rest of results.