Emailreg.org provides the Registered Email Send List (RESL) as a free lookup to anyone that would like to use it. It is also provided to those who have registered a domain here at EmailReg.org. The RESL is provided using a system that is similar to a DNS lookup. It is even more similar to a DNSRBL lookup. The RESL, however, requires looking up a DOMAIN and a IP address in order to determine if that IP address is a legitimate sender of email from that DOMAIN.
Append the following to the /etc/mail/spamassassin/local.cf.
header __RCVD_IN_EMAILREG eval:check_rbl('emailreg-trusted', 'resl.emailreg.org.') header RCVD_IN_EMAILREG_0 eval:check_rbl_sub('emailreg-trusted', '127.0.\d+.0') describe RCVD_IN_EMAILREG_0 Sender listed at http://www.emailreg.org/ tflags RCVD_IN_EMAILREG_0 nice net header RCVD_IN_EMAILREG_1 eval:check_rbl_sub('emailreg-trusted', '127.0.\d+.1') describe RCVD_IN_EMAILREG_1 Sender listed at http://www.emailreg.org/ tflags RCVD_IN_EMAILREG_1 nice net score RCVD_IN_EMAILREG_0 -100 score RCVD_IN_EMAILREG_1 -100
service spamassassin restart