I disagree with many of these opinions and assumptions. Research that I have done specifically on email and business contradicts several of these assumptions. Email is VERY IMPORTANT to virtually every business today. It is more important for user job performance than any other communications service. Yet it is among the least satisfying experience because of spam.

What doesn't work is the technology approaches that we have taken to date. Spam can be stopped from choking the inbox allowing users to do thier job. In the report 'The Problem with Email', Brockmann & Company introduced the Spam Index, a simple tool to help users determine how their experience compares to over 500 others. In a report to be released July 17, 2007 we compare Spam Index for 9 different technologies. Some are many times more effective than others.

Check it out: http://www.brockmann.com.

so Brockmann is still touting his flawed abusive Challenge/Response systems to all and sundry, and his equally flawed spam-index

well sir my own anti-spam tools {still in live beta}
a} allow individual users {per address}to choose which technologies they want to employ smtp-time
b} which blacklists they want to reject on {smtp time}
c} which other checks {ptr ehlo etc.} they want to employ smtp time
d} which hosts/domains/e-mail-senders they want to whitelist smtp-time

and thus far most have cut their spam to one every few days with, false positive level they are happy with.

with no use of abusive tech like Challenge-response or backscatter
{the reject message is all the challenge anyone should need, and offers a web-form {with capatcha to request whitelisting} while explaining all the reasons/tests the e-mail failed,so the sending system can be fixed to user acceptable standard.

when it goes public i will be releasing it via gpl to all

but anything is easier/better than challenge/response

I like this overview. I would only add, to update this since written, that many spammers are now crooks, and are trying to steal from you instead of sell to you. This means their tactics are to get you to click, just once, on a bad site or get your name and other info to rob from you.
Lotus Notes Spam