The Mailbox shares DN check box is used when two mailboxes share the same DN. You should clear this if what you are attempting to do is have two DN's share a mailbox. In this case, there is no second mailbox, all messages go to the primary DN mailbox and are retrieved from same.
Mailbox shares DN - one DN shares two unique mailbox numbers.
Two DNs share one mailbox - only one mailbox, one set of greetings.
30n30w