If your requirement is to enforce any unauthorized means of sending of mail, you can accomplish this by restricting permissions so that no one has permissions to send to anyone accept a few select mailboxes. These select mailboxes will have permissions to send to every user, and can be configured to forward messages to a corresponding distribution group. This is an advanced configuration with a variety of ways the permissions could be applied. Because changing the default permissions could have a negative affect, I would recommend that you test and tweak the permissions in a lab before attempting in production.
There is a straight forward way to restrict who can send inbound mail from the Internet to a specific user or distribution group. The Microsoft Knowledge Base article How to restrict the users who can send inbound Internet e-mail to another user or to a distribution group in Exchange 2003 documents how to set this up.
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