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.
Submitted by: Marcelo Araujo <email@example.com>