user mailbox of one Org can view the mailbox of another user mailbox

I was playing around few of mailbox i think i mess up with something. now org of one mailbox can view the mailbox of another mailbox. 
 
Can you please guide me a quick way of doing that. 
 

You can put ExtensionAttribute2 value as Organization Unit name after which the mailboxes would not be able to see other mailboxes.
 
Such issue occurs if you create mailboxes manually in Exchange (NOT using Hosting Controller) because manually creating Exchange mailboxes does not apply Address Book Policy on these mailbox. Therefore, always add mailboxes via Hosting Controller control panel instead of adding manually in ECP.