Spam has detrimental effects. Historically Internet mail servers have passed along email between third parties as a gesture of friendship and good will. Much like tossing your neighbor's paper over the fence if the paperboy misses his throw. This is happening less and less because spammers take advantage of it. Spam reduces the utility of email.
People become discouraged about checking their mailboxes if they are always cluttered with spam.
I decided to do something. Being the sort who favors technical fixes over legal ones , I started doing some research on the web , ordered a copy of the Bat book , and spent some time reading my sendmail configuration and scratching my head. I present here the result. First, if you're not using sendmail I can't help you. Second, you need the latest version of sendmail or these tricks won't work.