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BSD OSes: FreeBSD, NetBSD, OpenBSD, BSD/OS, Darwin (Linux,
Solaris, etc. should work too)
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Sendmail (techniques also applicable to other MTAs)
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DNS Blacklists
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smtpd/smtpfwdd (from Juniper)
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identd and fingerd (and replacements)
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IPFilter
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Sendmail "Milter" interface
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Procmail
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Open source "Filter kits" for spam and malware
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Log monitors
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"Anti-harvesting" techniques
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Teergruben
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Commercial/noncommercial virus checkers
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