Take a look at this Washington Post newspaper article:
http://www.washingtonpost.com/ac2/wp-dyn/A53496-2003May14
A few quotes:
"The spam problem is so bad that every network administrator uses some sort
of blacklist to sort good e-mail from bad, according to Nate Shue, a senior
network engineer at Vienna, Va.-based software firm Industrial Medium LLC. "
"Shue said the lists are more useful than spam filters because they block
offending e-mails before they reach the network. Blacklists are a more
efficient option than e-mail filters, which can keep most offensive e-mail
out of recipients' inboxes, but only after those e-mails have entered a
company's network. By the time the filter does its job, the recipient has
already paid the price of handling the message."
"Cohn fears that blacklists could be used by individuals or political
groups to stifle opposing views, since many operators rely at least in part
on customer complaints to compile their lists."
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