Description:

Active Spam Killer (ASK) protects your email account against spam
by confirming the sender's email address before actual delivery
takes place. The confirmation happens by means of a "confirmation
message" that is automatically sent to all "unknown" users. Once
the sender replies to that message (a simple reply will do),
future emails from that person will be delivered immediately. You
can also specify (regexp) addresses to be immediately accepted,
rejected (with a nastygram) or ignored. The package also includes
a utility to scan your old mailboxes and generate a list of emails
to be accepted automatically.