qmail greylisting

Three weeks ago I’ve set up greylisting on our little qmail toaster. The technique works amazingly well on our setup:
The server used to get hit by round about 1600 mails each day. Roughly 70% of those were spam mails. Now, with greylisting activated, this amount has been reduced to just a dozen spam mails and around 350 – 400 legit emails:

qmail AWstats - snapshot taken on October 23rd, 2006.

The server load is also reduced because SpamAssassin and ClamAV are less busy filtering mails:

Average server load - snapshot taken October, 3rd 2006 - four days after setting up greylisting. Peaks are running backups.Average server load for the year. One can pretty clearly see the difference the qmail greylisting makes on the server load.Average server load for the month - snapshot taken October, 23rd 2006. Again, peaks are running backups. The big peak in week 39 was the time of compilation and deployment. Thereafter you can clearly see the drop of server load compared to beginning of week 39.

So, speaking for us, this experiment has been very successful. Exactly no real mails got lost, tons of spam was blocked at the SMTP level. Excellent. Now, let’s see how long the effects of this technique will last…

See http://projects.puremagic.com/greylisting/ to learn more about greylisting.

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