HI
for the inbound emails u can modify each user from whom can receive emails using the active directory users and computers> specific user> "right click->properties> exchange general> delivery restriction from below group (messege restriction choose only from and Boss account to the list..
the outbound you can block it from the send connector, by creating send connector and modify the delivery restriction to reject by default except boss account
in this step you to use groups to apply for all users
hope this help..