I have been playing with the email filtering capabilities built into Netscape 4.78 with Netscape Messenger. (I tried 6.x versions of Netscape and didn't like them)

That's a shame because the latest versions of Mozilla have dedicated spam filtering built in.

Simple filters like the ones in Netscape 4 are not really going to cut it nowadays, unless you want to spend your whole life tweaking and adding to them. You really need a dedicated spam filtering system with filters that a bunch of people are working on, or a filter based on statistics.

If you really don't want to use Netscape 6 or Mozilla then you can use something like POPFile http://popfile.sourceforge.net/ which does spam filtering the same way Mozilla 6 does, using Bayesian stats to classify messages. I'm done with tweaking filters, using a filter that uses stats is much less painful, just the occasional bit of retraining (throwing a message back to the filter and saying "you got this one wrong").