Spamd
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spamd is an ISC-licensed lightweight spam-deferral daemon written under the umbrella of the OpenBSD project. spamd works directly with SMTP connections, and supports features such as greylisting, minimising false positives compared to a system that does full-body analysis. spamd is designed to work in conjunction with pf(4), and should be fully functional on any POSIX system where pf is available, i.e. OpenBSD, NetBSD, FreeBSD and DragonFly BSD.