Multiple people have mentioned similar ideas: procmail and a filter.

I just set up spamassassin (http://www.spamassassin.org/) a week or so
ago and it has correctly identified all spam to this list, and almost
all spam vs. non-spam I've received on the rest of my accounts, too.
It sets up very nicely as the first (filter) rule of a procmail config.

It seems to be the de facto standard for spam filtering these days.

On Mon, Sep 09, 2002 at 12:22:32PM -0500, Tony Shadwick wrote:
> Honestly, it may seem a bit presumptuous, but most of us are probably on
> unix mail servers.  It also seems obvious that most of us probably use
> procmail.
> 
> That being the case, might I suggest using Spambouncer?
> http://www.spambouncer.org.  Just add  /path/to/spambouncer/folder/sb.rc to
> your list of recipes, and turn on the switches you want and you're set.  I
> don't get spam.  Period.
>