There are a number of ways to filter out spam. Domain or blacklists are some of the most common methods of blocking unsolicited mail. These are lists compiled by ISPs, security vendors, and service providers that contain the addresses of known spammers. However, blacklists have become a contentious issue of late with larger organisations, who prefer to create their own custom blacklists.
IDC's Dahlgren says the main concern for businesses in regards to blacklists has been about false positives -- where a business sending out legitimate e-mails will be blacklisted. Unfortunately, once you are blacklisted -- even if unfairly so -- it has proved extremely difficult to be removed from the lists. However, Dahlgren says the concern for black lists has decreased.
"The problems companies have had with these lists have been diminishing considerably because there are certain characteristics of classic spam that are becoming known, such as the images in spam," Dahlgren says.
"Mail marshals working at hosts, such as Clearswift or Surf Control, will physically open spam e-mails and look at them to tag and categorise them all day long -- this helps them to create better criteria for their blacklist policies. This makes it very easy to filter out spam."
Other techniques are Bayesian analysis and heuristic filtering. Bayesian analysis looks at particular words, such as "Viagra", common in spam messages. Heuristic filtering is a way of scanning spam by looking at various aspects of the e-mail. Both the Bayesian and heuristic filtering methods are based on statistics and mathematical equations.
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