pgl4rbl implements a Postfix policy server that mixes two widely used techniques: greylisting and RBL (DNS blacklists). The idea is that greylisting is applied only to SMTP clients that match a RBL; this means that normal clients are not delayed (such as with a normal greylisting implementation), and RBL false positives do not cause problems (like when outright blocking them at the SMTP level).
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