SpamAssassin is an anti-spam solution that uses a wide range of different techniques. It was launched in 1997 and is led by the Apache Software Foundation, the creator of the famous Web Apache HTTP Server.
It is a very widespread tool among mail servers, including in the infrastructures of major suppliers. The technologies used include Bayesian analysis based on learned keywords, various blacklist systems (URIBL, DNSBL, etc.), content analysis (Razor and DCC) and a number of other message formatting criteria (image to text ratio, no text version, invalid originator, etc.).
SpamAssassin is available under the Apache License.
It is written in Perl.
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