From Wikipedia
E-mail spam is the most common form of internet spamming. It involves sending unsolicited commercial messages to many recipients. Unlike legitimate commercial e-mail, spam is generally sent without the explicit permission of the recipients, and frequently contains various tricks to bypass e-mail filtering.
Spammers obtain e-mail addresses by a number of means: harvesting addresses from Usenet postings, DNS listings, or Web pages; guessing common names at known domains (known as a dictionary attack); and “e-pending” or searching for e-mail addresses corresponding to specific persons, such as residents in an area. Many spammers utilize programs called web spiders to find e-mail addresses on web pages (see also address munging).
Many e-mail spammers go to great lengths to conceal the origin of their messages. They might do this by spoofing e-mail addresses (similar to Internet protocol spoofing). The spammer will modify the e-mail message so it looks like it is coming from another e-mail address. However, many spammers make it easy for recipients to identify their messages as spam by placing an ad phrase in the From field. Spammers try to circumvent the email filters by intentionally misspelling common spam filter trigger words. For example, “viagra” might become “vaigra”, or other symbols may be inserted into the word as in “v/i/a/g./r/a”. E-mail service providers have begun to use the misspellings themselves as a filtering test.
The most dedicated spammers�often those making a great deal of money or engaged in illegal activities, such as the pornography, casinos and Nigerian scam businesses�are often one step ahead of the providers. Retail e-mail services are updated constantly with improved spam filters, keeping track of spammers’ technological progress by examining e-mails their users report as spam (providers today have a prominent button to report spam).
So-called “spambots” are a major producer of e-mail spam. The worst spammers create e-mail viruses that render an unprotected PC a “zombie computer”; the zombie will inform a central unit of its existence, and the central unit will command the “zombie” to send a low volume of spam. This allows spammers to send high volumes of e-mail without being caught by their ISPs or being tracked down by antispammers; a low volume of spam is instead sent from many locations simultaneously.
Bill Gates, speaking at the World Economic Forum in Davos in January 2004, predicted that spam would soon be “a thing of the past”, and that Microsoft was working on several temporary solutions, as well as on a permanent “magic solution”, to spam (See “Solutions and Countermeasures” below).
