Don't you hate Spam? These days, Spam has become a major nuisance for anybody with an email account. Spammers harvest email addresses from various sources, including WHOIS records (for those who own a domain), HTML archives of mailing-lists, personal web pages and many other places. Those emails addresses are then used to send an unpleasant amount of equally unpleasant emails to all victims.
There are many traditional approaches to avoiding spam. The most common approach is based on "content-filtering": a program that attempts to classify incoming mails based on their content. This works partially, but in reality, it's somewhat easy to find ways around it.
ASK 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.
Still interested? There are some more details in How it works.