Spamresponder is a very simple and radical spam
filter, combining DNS validation with the
challenge/response approach. It will go through
all mail messages in /var/mail/$USER, looking for
specific "trusted" strings in the header and body
parts of the mail. It will delete and send an
automatic reply to messages which have no
"trusted" strings inside them. Those replies will
include one of the "trusted" strings, so the
recipient just needs to reply to this confirmation
message. The MailFrom-address of an email
containing a trusted string becomes a trusted
string itself.
