Exchange Server Based Rules vs. Outlook rules

Hi
I was wondering if it was possible to do the following and where I would do it, (Outlook ’03 or Exchange ’03):

When a user, (userX) has Send As permissions for another user, (userY) and sends a email as user, I want to store the sent mail in the sent items of userY rather than userX’s sent items.

Is this possible to configure? How would it be best done?

I would be grateful for any suggestions.
Regards,
chuck.