[Exim] How to force SMTP AUTH for certain return-path ?

    Finally I figured (thanks to Communigate Pro mailing list) what I'd
like to have to fight spammers that pretend to be somebody from our
domain. Is there way to reject messages without SMTP AUTH if return-path
header contains local address ?
Here is excerpt from Communigate Pro manual. I need the same for Exim.
Is it possible ?

-------------------8<---------------------------------------------------

This option can be used to force all "local" users to use the SMTP AUTH
feature. If the message Return-Path is an address of one of the
CommuniGate
Pro Accounts, the message will be rejected if the client mailer has not
sent
the SMTP AUTH command first.