In a previous column I suggested - somewhat tongue-in-cheek - that the answer to spam was to levy a microscopic tax on each email sent. Not surprisingly many readers spammed - I mean emailed me - to tell me how wrong I was.

For the first time in my life I understood what it feels like to be a politician - if you really want to be noticed talk about introducing a new tax.

But the rejection of my plan is good news for anti-spam software vendors who offer a growing armoury to fight the problem.

The latest must-have is challenge-response technology, which is the real sledgehammer of anti-spam tools. It works by building a whitelist from your address book. Arriving email is checked against this list and if it is from an unknown source it gets an auto-reply asking the sender to verify their identity via a web site or by reply email. Spammers hate it because it needs a manual reply to get through and so is nearly 100 percent successful.

The problem of course is that this "Do I know you?" approach is cumbersome and off-putting to newcomers who email you. It's hardly appropriate if you are hoping to encourage new customers to get in touch. But the beauty of challenge-response is that it can be scaled down so that at the very least certain emails can be quarantined for manual checking, after other filters have removed most spam.

The second breakthrough is the growing strength of collaborative filtering.

I've just tested Cloudmark's SpamNet and found it 95 percent effective. Using peer-to-peer technology, it continually gathers details about spam from all its users - currently standing at 650,000.

If you install SpamNet your email is checked against the SpamNet database, which learns from every bit of spam you or others detect. False suggestions are weeded out by a proprietary voting system, which qualifies spam based on the number of user votes and the history of each subscriber as a trusted user. This technology has just been adopted by Sendmail, by the way.

While politicians continue to chase spammers by implementing tougher laws - that are all too easy to get around if you have an offshore server - it looks like challenge-response and collaboration filtering offer the best hope yet.

Of course we all end up paying in the same way we do for antivirus software; and we end up supporting an industry that, perversely, profits each time the spammers find a new way through. Anti-spam software now has the ability to kill 99 percent of spam - but at a cost. The problem for governments is how to protect the millions of people who can't afford it. It's high time Microsoft stepped in and made Outlook Express filtering far more effective.