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Absolute FreeBSD, 2nd Edition by Michael W. Lucas

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Greylisting

Blacklists provide unilateral protection against known spam sources. Whitelists document known good email sources. The key word in both of these statements is "known." Stopping unknown threats is much harder than stopping the known ones. Greylisting provides a decent middle ground that lets you reject known bad email and accept known good email, while helping you sort everyone else into one of these categories.

Most junk email sources try to deliver as much email as possible to as many recipients as possible. To do this, they use a stripped-down version of the SMTP protocol. In particular, most botnets don't try multiple times to deliver email. Real mail servers will try for up to five days to deliver a legitimate message, while botnets ...

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