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Email Server Greylisting and Spam Filtering
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eddie
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Joined: 27 Nov 2001
Posts: 3373
Email Server Greylisting and Spam Filtering  Reply with quote  

I apologize for not posting information sooner, but I have been out of the office for the past 10 days. The proper information about changes was not sent to Ilion so that he could post in my absence.

After working with "Spam Assassin" for 1 week, it was determined that it would not fit our needs. The sheer amount of email that we receive and send every hour is well over 100,000 emails. Given that Spam Assassin is designed to scan every email that arrives and assign it a rating on the probability of that email being spam, rewrite the headers to include this new info and then send the email to the real mailservers resulted in too much CPU power being required. So much CPU power was required that we would have to dedicate around 30 servers for just Spam Assassin processing.

Instead, we have started a new method of spam control called "Greylisting". Greylisting uses existing SMTP protocols that greatly reduce the amount of spam received. How this works is when a mailserver connects to our servers to deliver email to our customer, our mailserver keeps track of the from address, to address and IP address in a database. If this from, to and IP address have been processed before (meaning the same person has sent you email before from the same ISP and email address and it has not been over 30 days since they have last sent you an email), the email is delivered immediately. If this is a new email, our servers then tells the sending server that there is a temporary error and to retry. The retry time is dependant on the sending server.

We are working on a way to allow the users to disable this Greylisting for those who do not want it, but if spam continues to increase at the rate it has, spam filtering will exist everywhere and be mandatory everywhere.

We have also recently increased the size limit for sending and receiving email attachments. The limit was 5 megabytes before. It has now been increased to 10 megabytes.

For much more technical detail that I can provide here, you can go to http://projects.puremagic.com/greylisting/whitepaper.html .
Post 08-24-2004 04:08 PM
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