Gone Grey Listing for SPAM

Grey Listing - Perl Implementation

I have added a grey listing feature to the e-mail server for my hosting solution. Grey listing is a fairly novel idea that has reduced the CPU load from ~70% to less than 10%. Which is a wonderful thing, let me tell you.

Grey Listing works on the principle that spammers use a 'fire once and forget' method, while servers sending legitmate e-mails are going to be persistent. Now when my server recieves a incoming smtp connection from a 'new' IP, it simple responds with an 'I am to busy, come back later' message. The greylisting e-mail server will continue to tell the new IP sending server the same message for a period of time; in my case 10 minutes. Then if the after 10 minutes the smtp connection will be accepted, and the e-mail will be processed. The sending IP is now 'approved' to send messages without delay for 6 hours; the counter is reset everytime a new message is recieved. Once the 6 hours is up, and no message has been recieved, the sending server is required to wait the ten minutes again.

I personally think this is an ingenius scheme. Spammers don't write MTA's that are going to patiently wait for 10 minutes just to send one message. Its simply not worth their time. So the fire-once-and-forget spammers now see my server as 'always overloaded' and the patient good citizen e-mailers do as they have always done, and are delayed by one 10 minute period. However of course this means that you won't recieve your e-bay approval messages, and first weekly status reports immediately. I am researching how long an approval time should be. That and giggling at the fact that I've already rejected over 12K spam messages in under 30 minutes.

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