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 inetd.conf (or
tcpserver) invocation.

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://pobox.com/~djb/rblsmtpd.html for info and updates.

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

-John Saunders <john@nlc.net.au>