Fight spam with greylisting

The new server has now been live for almost a week and we actually switched over the mail one day before we turned the entire site live. One thing that really has improved for the people having mail accounts here at OS2 World - is less spam - way less!

Having spam filter is one thing, but the question is still, should just mark the incoming e-mail as spam and forward it and then leave it to the user or should be delete all incoming spam? Just deleting spam generates another problem that I often have had when communicating with companies that uses spam filters; the spam filter often triggers on IT security related terms and quite often I had to check with the people if they really got the e-mail or not.


So when talking around within the OS2 World staff and how to handle spam, Tommy Jansson - our sysop, mentioned that we could give it a try to set up a greylisting function that takes the first hit of all incoming e-mail.


So how does greylisting work? What happen is that each time a given mailbox receives an email from an unknown contact (ip), that mail is rejected with a "try again later"-message. This, in the short run, means that all mail gets delayed at least until the sender tries again - but this is where spam loses out! Most spam is not sent out using RFC compliant MTAs; the spamming software will not try again later.


If we take a look at the last 6 days that the server has been up and running it has so far rejected 30.885 e-mails (I actually didn't think that we received that much spam!).




So if we look at the next picture we can there se that we have in total during the last 6 days received 1661 e-mails where 6 has been cleaned out for virus and 88 was marked as spam. If I compare this to one of the other mail servers that I have and where greylisting isn't in use (yet) the number of e-mails marked as spam is way higher, but still sent on to the users where Thunderbird does an amazing work to clean out spam. Still, I receive data that I don't want and greylisting seems to work fine so far. At least it gives us hopefully a few extra hours to identify spam and get the spam filters updated.




If you are wondering about the blue peak on sent e-mail from the server - this when I sent out an e-mail to around 1700 e-mail about the server update earlier this week. But I hope that this little greylisting test might give some of you an idea how to fight spam.

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