Why does the spam filter return "unsure"?
If Mollom is not certain whether a post is ham or spam, it will answer "unsure," and we present the user with a CAPTCHA challenge. A human user can easily solve it and the content will be accepted, while automated spambot scripts will fail this test and be blocked. This "gray zone" allows us to use much stricter content filtering and still be sure that human users will be able to post their content. See this "The science behind Mollom: Spam vs. Ham" blog post for more information.