[Exim] SpamAssassin for all mail

We have a couple of mailhubs which look after all mail deliveries whether
it is to local mailservers or to external sites.

The SA configuration of EXIM3 works fine for local deliveries but I would
also like to scan external deliveries as we don't want to send spam from
this site (in theory).

Any ideas on how to do this.

John Linn