From Marco Paganini:
ASK (Active Spam Killer) takes advantage of the fact that most spammers use invalid or fake "From:" address in their messages. When a new message arrives and the sender is unknown, ASK sends a "confirmation message" back, informing the sender that the original message has been queued, pending confirmation. When the sender confirms (a simple reply), ASK delivers the original message and adds the sender to a "whitelist". Further messages from this sender will be immediately delivered. It is also possible to ignore messages based on specific criteria, like sender's email, subject and so on.