rblsmtpd is a generic tool to block mail from RBL-listed sites.

It works with any SMTP server that can run under tcpserver; in
particular, any version of qmail or sendmail. Turning it on is
easy: simply insert rblsmtpd in front of the real SMTP server in
your tcpserver invocation (or inetd.conf).

rblsmtpd supports anti-RBL lists for sites that want to skip RBL lookups
for preauthorized hosts. It also optionally pays attention to temporary
RBL lookup errors. 

See http://maps.vix.com/rbl/ for more information about the RBL. 

WWW: ftp://koobera.math.uic.edu/www/rblsmtpd.html