What's a challenge and response spam filter?

You've bumped into a challenge/response spam filtering system. The logic behind these is good: if you want to get a message to my inbox, prove to my little robot assistant that you're a human, not some insidious spam application trying to waste my time and resources.

How to do this easily? Well, you can't just send out a response message and ask for a reply, because that can be easily recognized and automatically answered, defeating the purpose of the spam filter in the first place. You could embed one of those weird "letters floating in space" decoding graphics and ask the user to figure out the response, but having an actual input form buried in an HTML message is too much hassle.

So how about a graphic that has a visual puzzle and a set of buttons that allow you to pick the correct answer?

That's exactly what MailFrontier does, among others, and it just so happens that I have a message from them in my own mailbox compliments of my colleague Brad Fallon. Here's what the graphic in the message looks like:

Does this kind of system work? Yes, but. The problem is that lots of people -- myself included -- get too much email already and aren't willing to respond to challenge response systems. If your circle of colleagues and acquaintances are willing to jump through these hoops, then it might be a good solution for you, but if not, well, you'll be missing messages without ever knowing about it.