We suspect a user has granted access to their mailbox via delegate rights, or right clicking their mailbox > folder options > permissions > add.
From some testing, the shell commands i.e. get-mailboxpermissions do not seem to show permissions set this way. Is there any way you can retreive this information from a shell command or other powershell script? it looks like there are 3 ways to set permissions to a mailbox, 2 via an exchange admin and 2 via outlook itself in delegates i.e. file > account settings > delegate access, and the other way right clicking a mailbox in outlook > folder options > permissions.
if get-mailboxpermission is only showing permissions set by an exchange admin, I need a way to list permissions set by the other 2 techniques, to get a full list of who can access their mailbox. Logging into their mailbox via AD/Outlook is not an option at this stage.