Blacklisting, content search for SPAM, electronic mail, greylisting, SMTP, SPAM, unsolicited e-mail messagelacklist- ing, content search for SPAM, electronic mail, greylisting, SMTP, SPAM, unsolicited e-mail messages

Permanently changing nature of SPAM (hereinafter only in the sense of unsolicited e-mail messages thus intentionally neglecting other SPAM types) causes that it is almost impossible to find any single mechanism protecting against SPAM for long- term periods. Multi-layer protection is usually applied to SPAM elimination therefore. Especially blacklisting and greylisting have been proven to be extremely useful because they can eliminate significant part of SPAM messages even before their delivery. The article gives an overview of existing SPAM elimination methods, motivates and describes multilayer anti-SPAM mechanisms and analyses the behavior and the efficiency of the key components of a multilayer system. Weaknesses of the analyzed methods (especially blacklisting and greylisting) are mentioned, too, and recommendations formulated, both based on the author?s own measurements.