If previously it was relatively easy to block spammers, since they used to spam from a handful of IPs, the new junk mail prevention technologies made them abandon this approach, as they were getting blocked as soon as they would start sending junk email. The new solution used by spammers is the botnets. Botnet is a cluster of computers that were broken into ("owned") by a person or via a virus attack (usually mounted via email). A program is installed on all these computers, which now control these computers remotely - therefore those machines are usually referred to as zombies.
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