Paul Vixie is involved in a project
called the Mail Abuse Protection
System (MAPS). He is publishing his list of sites which engage in, or
tolerate abuse of email. This Real-time Blocking List (RBL) is
available, well, in real-time. This list is available as a BGP
feed, or through the DNS. Dan Bernstein has a fake SMTP server called
rblsmtpd which consults
the RBL and traps accesses by RBL'ed hosts.