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policyd-weight is a weighted rejection-policy daemon for Postfix. Its website is at www.policyd-weight.org. The Gentoo package is mail-filter/policyd-weight.
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
HELO/EHLO-claimed identities versus
MAIL FROM (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.
emerge --ask mail-filter/policyd-weight
Further configuration is necessary.