You can specify which recipients a mailbox, distribution list, or custom recipient will accept messages from (not available for public folders). All rejected messages are returned to the originator.
You can accept or reject messages from any sender listed in the directory. Delivery restrictions are optional and do not affect incoming messages. The default is to accept messages from all senders and reject messages from none. If you specify even one name in the Accept messages from box, messages from all other senders who are not listed in the box are rejected.
Note If a distribution list is specified, the members of the distribution list are used for the restriction, not the distribution list itself.