[Exim] Protect again spam

Hello together,

i am looking for a way to protect against spam.

We have some mail-realy systems, based on exim. To block spam, we use now
filter-rules based on "subjecs" and some other fields.

This works in some kinds, but is not really (in our config) effictive.

Are there other ways to protec again spam?? I think that
commercial-solutions will be OK when they work together with Exim.


Reguards
bjoern