You have to remove the leftperson@yourcompany.com SMTP proxy from the left person's mailbox before you can add it to the boss's mailbox. And this will only catch incoming external email, of course. To have all the replies to internal mail the left person send go to the boss's mailbox, you need to add the Object Distinguished Name of the left person's mailbox to the boss's mailbox as a secondard X.500 proxy, and then delete the left person's mailbox (I'd take a PST of the mailbox first, and offer it to the boss so that they have access to all the mail after you delete the mailbox).