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Robert Cringely’s latest article presents an idea to prevent spam: charge a nickel a message. Others have suggested this before. My response summarizes the approaches to spam prevention and where this one fits in. There are two basic types of anti-spam solutions:

1. Putting the burden on the recipient. (SpamAssassin, content-based filters, DNS blacklists).

   • advantages: No special effort for senders, no barrier for innocent first-time contacts
   • disadvantages: There are always some number of false negatives and false positives.

2. Putting the burden on the sender. (TMDA, Cringely’s nickel-a-mail idea, etc.)

   • advantages: Can block 100% of spam.
   • disadvantages: Takes effort (or money!) to reach you. Most people won’t bother and you won’t hear from first-time contacts. Problematic for automated mail you want, like mailing lists or online order confirmations.

Cringely’s idea, which we’ve heard before, is in category 2. So you have to compare it to existing sender-burden solutions like TMDA. TMDA is a spam filter that includes, among other things, a system that sends a confirmation message and requires the sender to respond or click a link to complete the delivery.

In this comparison TMDA wins hands-down: it requires no special software or account on the sender side, no special account with Paypal or anyone else on the receiver’s side, no infrastructure that doesn’t already exist, and it’s free. And it’s every bit as effective as the nickel-a-message system.

Even if spammers create an automated way of responding to TMDA confirmation messages, it (a) requires them to have valid email addresses, (b) requires twice the time spent sending mail, and (c) can be easily defeated with a CAPTCHA or other approach. Why bother charging money when this works?

Of course, nine times out of ten when I get a TMDA autoreply, I just delete it. I’m usually trying to help someone with my message and now they want me to jump through hoops for the privilege of speaking to them. No thanks. That’s why I don’t use TMDA on my own mail – I’m not famous enough. If I was Robert X. Cringely, it would be the perfect solution.