Postfix/policyd-weight

policyd-weight is a weighted rejection-policy daemon for Postfix. Its website is at www.policyd-weight.org. The Gentoo package is.

Compared to standard email server configurations, it intercepts mail before it hits the MTA for queuing, is capable of querying RBLs and verifying sender IP addresses versus SMTP -claimed identities versus   (sender) addresses and generating a weighted email HAM vs. SPAM (ie. legitimacy) score. You can set a threshold value beneath which mail is rejected, thereby saving CPU time and resources. You can see a diagram of the approach at the policyd-weight website.

Installation
Further configuration is necessary.