View Source: GreyListing - Waikato Linux Users Group

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GreyListing is a new technique for reducing the volume of [Spam].

!! About Greylisting

Greylisting relies on the inherent reliability built into the [SMTP] protocol - basically, you can temporarily reject a message and be reasonably confident that the sending [MTA] will hold on to it and retry later. The idea is that the software spammers use isn't smart enough to deal with temporary rejects - bulk throughput is better than technical correctness in their case. Such software will never reattempt delivery, however real MTAs will.

Incoming mail is identified by a triplet of data:

* The IP of the sending host
* The envelope sender address
* The envelope recipient address

The first time your MTA receives an email from a specific triplet, it sends a temporary reject notification to the sending MTA. Any real MTA will obey this temporary reject, and resend the email after a delay period.

There are some other modifications to the above description, such as:

* If more than one triplet arrives within the first timeout, delay it as well.
* If mail for a given triplet hasn't been observed for a significant period of time, remove the records relating to it.

!! Implementations

* http://www.maretmanu.org/homepage/inform/exim-spam.html
* http://greylisting.org/implementations/

!! References
* http://www.greylisting.org
* [Greylisting Whitepaper|http://greylisting.org/articles/whitepaper.shtml]
* http://www.acme.com/mail_filtering/introduction_frameset.html
* http://projects.puremagic.com/greylisting/
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Last edited on Sunday, June 19, 2005 10:31:16 pm by DanielLawson

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