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Using Mollom in Drupal 7 installation on contact form:
When captcha is triggered successfully completing the capture returns message "email has been sent" but no email is sent. If the content of the message is changed to something less suspect the message does get sent. So when and "uncertain" is flagged by the content of the message and captcha is triggered if the message remains unchanged it seems then to be blocked even though the site response message says it is sent.