Junk Mail Buffering Agent is a tool for preventing delivery of email until the
sender has confirmed their address. It can be used in conjunction with a spam
filter to prevent delivery of spammy-looking email unless the sender is
confirmed as valid.
jmba is designed to be used in conjunction with a spam filter such as QSF and
the mail processor procmail. When the spam filter says it thinks an email is
spam, it can be passed to jmba. jmba will queue it and send an email to the
sender containing a key; if the sender replies, the original email is
"unfrozen" from the queue and delivered.

WWW: http://www.ivarch.com/programs/jmba.shtml