SPAM represents more than 90% of SMTP traffic in the Internet. Greylisting has become one of efficient tools for SPAM elimination since its proposing in 2003. So far only few studies focusing to greylisting efficiency were published. Greylisting implementation in Postgrey has been successfully used in the Author's university SMTP server since 2006. There were some attempts to evaluate its efficiency based on logs but due to many other aspects of SPAM elimination playing the role the direct efficiency comparison between various periods is difficult. Therefore also other SMTP servers at cooperating bodies were examined to obtain proper information about its efficiency. Two SMTP server using Postgrey with Postfix and one SMTP server using greylisting in different implementation were studied; the efficiency was estimated and mutually compared. The main result of the study is that significant efficiency decrease was not observed despite greylisting simplicity. This study extends previous findings regarding efficiency to longer period and what is especially significant, confirms results by comparing results to other sites. Also another finding demonstrating the input Postfix filtering as an efficient anti-SPAM measurement is illustrated. The main conclusion is that filtering SPAM as soon as possible, i.e. before real delivery of SPAM messages to target SMTP server, is extremely efficient anti-SPAM measure comparing other methods.
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