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://www.acme.com/mail_filtering/introduction_frameset.html
• http://projects.puremagic.com/greylisting/

Part of CategoryAntiSpam