Aligning Yourself With Block Lists - Back to the Blacklist
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There are lots of black lists, and like I said, ESPs and ISPs use them as a "buddy" system. If an email is sent to an ESP or an ISP, it may decide to check with its own "buddy" DNSBL, which it uses to check whether the IP address has been reported by another service provider as a source of spam. The blacklists (or block list) such as Spamhaus also have their own active monitoring processes for discovering spam email (mostly through the use of spam traps and cooperative service providers).
These lists only serve as a reference to service providers or receivers and are the "blockers" themselves. Their major strength lies in the fact that service providers trust them and their reporting system. A lot of black lists are not well maintained and do report wrongly. Because of this, sometimes you can be blacklisted and be doing nothing wrong! Currently being blacklisted is not a big deal, and it can be a mistake depending on the list. However there are some highly reputable ones; being blacklisted by them could mean you are engaging in active spamming. Some examples of such top black lists include www.spamhaus.org, http://www.spews.org/, www.dsbl.org and the very first blacklist, MAPS (Mail Abuse Prevention Systems).
The Spammer
Once the blacklists the service provider uses confirm that the IP address sends spam, then, depending on the policy of the service provider, the service provider either moves the spam to "junk" mail, does a deeper heuristics of the email or (a lot of small service providers) bounces it back to the sender. The "spammer" starts getting bounced mails (continuously) with unique messages stating you have been blocked due to a listing (depending on the particular site administrator's policy).

HOW BLOCKLISTS WORK (Courtesy of http://www.spamhaus.org/ )
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