Challenge Response Anti-Spam for Exchange Server and Lotus Domino

Policy Patrol Spam Filter for Exchange and Lotus Domino includes a challenge-response system that allows you to quarantine emails from new senders and send an auto reply asking the sender to verify their email message by following a web link. When the sender enters the text in the captcha and clicks OK, the message is automatically released from quarantine and the sender email address is added to the white list.