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A natural reaction to receiving spam is a desire to make sure that no more arrives from its senders. This has led to the creation of blacklists: IP addresses that are blocked by participating Internet providers and others to ensure that no e-mail can get through from those using them.
One of the best-known players here is the specialist not-for-profit organisation Mail Abuse Prevention System ( Maps). This puts together the Realtime Blackhole List, a constantly updated list of spam-friendly networks, available to subscribers for blocking.
As the background information about Maps indicates, one of the people behind it is Paul Vixie, who has been one of the key programmers for the main Internet domain name software Bind. The Maps blacklist is really designed as a last resort last resort in the long and rather tiresome process of trying to stop spam by other, more direct, means.
A similar outfit is the splendidly named Spews.org/, which offers its blacklist for free. There is a FAQ and also fascinating news of spammers and action against them. The Spews.org site is particularly useful for its list of spam advisory services, spam filtering systems and other interesting sites connected with spam that appear down the left-hand side of the home page.
Unfortunately, such formal blacklists are less efficient now that spammers have become more widespread and more technically savvy. It takes time to track down the origin of spam; meanwhile, millions of junk e-mails flood into the system - and everyone's in-boxes.
There are plenty of companies that try to help solve this problem. Brightmail claims to be the anti-spam leader. It certainly offers plenty of interesting material on the subject. Its basic approach is a clever one. The company actively seeks spam, using dummy e-mail accounts, which it then characterises before sending on the details to its spam-blocking software installed on end-users' servers. These can then sort incoming e-mail according to the various updated rules that have been generated by Brightmail's analysis.
Although not really aimed at the corporate market in the way that Brightmail is, also worth noting is the popular Spam Assassin. The original version was for Unix systems, but there is now something called Spam Assassin Pro for Microsoft Outlook.
Spam Assassin's spam identification methods draw on many approaches. It analyses e-mail headers and content to spot spam, taps into blacklists, and also makes use of Vipul's Razor. This is a collaborative filtering system that depends on thousands of users sending in their spam reports. Special statistical rules minimise the risk of addresses getting blacklisted by mistake.
In some ways, Vipul's Razor is a variant on Brightmail's approach, but one on a larger scale that draws on the core strength of the Internet - its users. The creator of Vipul's Razor has also set up a company called Cloudmark to produce Unix and Windows products based around this idea.
The complementary approach to blacklists is to use whitelists. As its name suggests, this is the conjugate of a dedicated blacklist, and effectively only allows through e-mail that has been explicitly sanctioned by the user. The Tagged Message Delivery Agent is one way of providing this.