Never tried Mailwasher...but...I did buy McAfee SpamKiller first and I HATE
IT. Right in the garbage. You have to get your mail through it and then
pick out what you like and don't like. It's poo.
Try one of the Bayesian filters (spambayes gets good reviews) for a
couple of weeks, and I bet you won't go back to spamkiller. I just
upgraded Eudora to v6.0, which has a bayesian filter built in, and
quickly dumped SpamKiller. Night and day.
Most of these filters, like spambayes, can work as a pop proxy, or as
an Outlook plug-in.
http://www.paulgraham.com/spam.html is a great place to read about
this relatively new technology (or more accurately, relatively new
application for an old theory). He also has a page listing all of the
software using this technique.