Apr 25, 2006
Challenge-response spam filters

Whenever you send an email newsletter or promotional campaign, you get all kinds of bouncebacks.
That’s why it’s really important to use a reply-to email address that a human in your company actually checks on a regular basis. Especially the few minutes immediately after clicking "Send."
We just sent our MailChimp Newsletter, and within 10 seconds got half a dozen "I’m on vacation replies."
But mixed in with those autoresponders, we always find one or two "challenge-response" spam filter replies. If you’re new to email marketing, and you’re not familiar with challenge-response spam filters, read on…
Here’s how challenge-response systems work.
You send someone an email. His spam filter says, "Hmmm, I don’t recognize you." The spam filter sends you back a reply with some kind of question, like "how many puppies do you see?"

If you answer correctly, it proves you’re a human (and not some kind of spam-bot), and you’re added to the recipient’s "approved" list. Once you’re on the list, you never have to do it again.

Here’s some information about one popular challenge-response system, MailFrontier Desktop.
There are other challenge-response systems that require you to answer a question, but then they also give you an opportunity to send a brief message to the recipient, so he’ll be reminded of who you are (Earthlink uses this approach). For example, I once met with a potential client, then sent a followup email after the meeting to thank him for his time. I got an Earthlink challenge-response email, asking me to provide a little explanation of who I am.
So you really can’t just blast out a campaign and walk away. Make someone responsible for checking the bounces, and be sure to tell them how to respond whenever they get a challenge-response email.
jason
yes, but what do you do when you send out multiple mailings that go to thousands of people? “checking the bounces” could be a full time in and off itself in that case…
04.25.2006
Ben
Amen to that. Whenever we’ve handled large campaigns on behalf of our clients (with huge lists), we got all the bounces. And yeah, it can get outta hand fast. But it’s nothing a few inbox rules and folders won’t solve. It’s just important that the bounces go to a real inbox that a human can review. We can’t read every word of them, but the challenge-responses are pretty easy to spot (look for the puppies and kitty cats!).
04.25.2006
Tomasz Andrzej Nidecki
It’s even more fun when you get a huge amount of such confirmation request from people you never sent anything to. The wonders of e-mail spoofing… That’s why, until we get rid of e-mail spoofing [and we probably never will], challenge-response unfortunately should be abandoned. I abandoned it strictly for that reason.
04.26.2006