Spam and Open Relay Blocking System (SORBS) was originally an anti-spam project where a daemon would check, in real time, all servers from which it received e-mail to determine if that e-mail was sent via various types of proxy and open-relay servers. SORBS has evolved into SORBS DNSbl (DNS-based blacklist) which now includes more than 3 million listed hosts that are considered to be compromised (Web servers which have vulnerabilities that can be used by spammers).
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