[Exim] spam with forged from:

    Hi everybody,

I need to solve the following problem.
Some spammers use our company e-mail addresses in From: and To: fields
of their messages.
As you understand our mail relay allow relaying to everything that sent
to our domain.
We have SMTP AUTH in place, but it does not help in this case since it
is only being triggered when the To: field is something non-local.
I'd like to set up Exim to force SMTP AUTH in case when From: AND To:
headers contain our local domains. Is that possible ?

    TIA.