Take a look at http://www.vix.com/spam.  I have actually hacked a list
of sites which send me more spam than useful information, and I refuse
email from those sites, but this doesn't eliminate spam - it just gets
rid of the easy stuff.  If they forge their headers, sendmail believes
them.

You can also get a daemon called smap which is a proxy SMTP server for
firewalls.  It can be programmed to refuse email from certain groups
of IP addresses, and to look at the email in somewhat more detail than
sendmail does.  I haven't had a chance to check it out yet.  It was
part of the TIS firewall suite, so you can probably find it somewhere
on ftp://ftp.tis.com.

			       _MelloN_