If you are hosting multiple domainnames with emailaccounts within an Office365 Exchange account, each emailaccount can see all other emailaddresses through the “all users” addresslist. This is a nice future if the domainnames belong to one organization but it isn’t if the domainnames belong to different organizations. To avoid this you can disable the visibility of an emailaccount in the addresslist.
Go to the Exchange admin center:
Click on recipients -> mailboxes
Select the user you want to hide and choose Edit. Now select “Hide from address lists” and save the user.