[Exim] How do I filter spam earlier in the delivery?

Hi,

As an Exim (and MTA/MDA) newbie, I hope my question makes sense, and
uses the correct terminology...


I've tried to implement a system anti-spam filter. For example, I fail
messages with the header field X-YahooFilteredBulk. However, it seems
to me that this just moves my spam problem from my inbox to my message
queue (now I get lots of error messages frozen due to incorrect return
addresses). I could just silently dump the message, but I would prefer
to send some kind of failure feedback.

It seems to me that the system filter is applied after Exim has accepted
a message for delivery. This seems too late in the process - I don't
want to take responsibility for these messages. I would much rather
return 550 (or whatever the code is) to the client MTA instead of 220
and not accept the message at all, and make them deal with error. Is
there anyway of filtering at this earlier stage?

TIA
Malc