Have you ever sent an email message, and received an automated response in reply? And in that reply, you are asked to “prove that you are human” by clicking on a link and/or entering a CAPTCHA code. What is this? Is it new?
When that happens, you’re interacting with a “challenge response” email filtering system. When you receive a “prove that you’re human” reply, that message is a “challenge” that the spam filter is requesting that you to respond to. This “response” to the “challenge” helps the spam filter (in theory) know that a real person sent the original message.
It’s not that widely used, nor is it that widely loved, because it has a pretty big flaw. Very little spam has legitimate from address on it. Most of the time, the from address is forged. It goes back to some innocent, unrelated party. In those cases (i.e. “for most spam,”) the challenge email is sent to the wrong person. So, you end up spamming unrelated people with “challenges.” Ever received a challenge request in reply to an email you never sent? Yup, that’s what’s happening. It’s just as bad as the spam itself, in my opinion. It’s an annoying email, probably sent in bulk, to people who didn’t ask for it.
Occasionally marketers freak out, thinking, “OH MY GOSH! MY MESSAGES AREN’T GETTING THROUGH!! THEY’RE GETTING TRAPPED BY THESE FILTERS!!!” That reaction is overkill. Don’t freak out! This kind of filter is not widely used — and it is not new at all. Heck, just about four years ago, I helped to answer a challenge/response question for Email Insider’s Email Diva column.
I guess this is one of those things that comes up again periodically, because there are always new people in our industry who haven’t stumbled across it before.
An industry colleague of mine, who works for a major ISP, was asked what he makes of those filters. “It is what it is,” he replied. Meaning, perhaps, that these filters are not great, but there’s not much you can do about them, and they are really not worth losing all that much sleep over.