[exim] SpamAssassin transporter - mark email as read

All my mail is currently going through a SpamAssassin spamcheck router
and transporter and then being passed to a SpamAssassin command which
in turn passes it back to exim to start over. Obviously exim has to
make sure the check hasn't been done.
I want to detect when it has been marked as spam and pass it to a
different appendfile transporter, how would I do this?

Many thanks

Craig