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.
Source: Github - Microsoft
Elevation of Priviledge
MITRE ATT&CK Tactics
Regular - sanctioned operations activities
If your organization has valid admin, delegate access given to users, you can whitelist those and investigate other results.