Recent reports indicate that spam is increasing again.  I have been using Exim to filter spam for several years.  Some recent tuning I have done have decreased the percent of spam which reaches my spam filters.   This article provides a discussion of the techniques used, and provides implementation examples.   Spambots tend to be simple programs which don’t handle slow servers very well.   Using a greylist is effective method of blocking them as they usually don’t retry.   My latest changes use delays to cause many spambots to abandon their attempt.  Greylisting is used only for poorly configured servers that make it to the Recipient command.

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