What is a challenge/response spam filter?
When an incoming email arrives in your inbox, the spam filtering service hides it from you. It sends an automatic reply to the “From” address of the email, requesting that they respond to verify that the email sent was not spam.
If the email is spam, the “From” address is likely invalid. Even if it is valid, automated spam systems presumably are not smart enough to do the response required by the filtering service. Thus, “100%” of spam is eliminated.
List of Challenge/Response Spam Filters
As the owner of GoogleCommunity.com, every time I send a newsletter (which currently is very rarely), I get lots of challenges back. Since my email isn’t spam, and I get only a manageable number of these challenges, I typically go through and manually respond to each.
There are many such systems, and sometimes, when quickly going through email, I miss one. So yesterday I spent nearly an hour frantically trying to figure out the name of the spam filtering service, so that I could search my Gmail inbox for their challenge and respond appropriately.
Here’s my list:
- BoxTrapper (by CPanel)
- ByteFortress
- T324 SpamSpoiler
What do you think of challenge-response spam blocking? Do you use one of these services, or another one?