[exim] How do I block senders at SMTP? (MAIL FROM:)

Hi,

How can I block senders via their "MAIL FROM:" input and give lets say 4xx
or 5xx at the SMTP communication level?

    > EHLO asd
    < 2xx xxxxxxxx
    > MAIL FROM:<from@???>
    < 5xx xxxxxxxxx...

PS1: I know this could be against some RFC yadda yadda's.... :)

PS2: domain1.dom is not a local domain.