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.