This graph shows the effect of our antispam measures. Prior to week six there was no antispam on this server and so the graph is at all zeroes. At week six, we added spamassassin, and you can see the blue line, representing total mail, diverge from the green area, representing caught spam. At the beginning about 75% of incoming mail was being marked as spam. Our false positive rate (mail marked as spam that wasn't really) was < 1%, and our false negative rate (spam not marked as such by spamassassin) was about 10% of accepted mail, or about 2.5% or total mail volume. Although our users configure their email clients to thrown the spam into its own folder, scanning that folder for false positives still took up several minutes a day for the typical user.

At the end of week seven, we added greylisting. This causes incoming spam to be rejected completely and has a zero false positive rate. It can also tie up the server that is sending spam. As a result, spamassassin has quite a bit less spam to detect. Currently only about 4% of incoming mail is marked as spam! (This is as compared to the 75% seen before). This means that scanning the spam folder for false positives takes only a few seconds.

In conclusion: this domain had a serious spam problem, with over 75% of incoming mail being spam. This has been reduced to less than 4%, and even that small amount is marked as spam and moved to a separate folder where it can be checked for false positives once or twice a day.
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