The magazine of the Melbourne PC User Group Improved Spam Management with Grey Listing Roger Brown

Members will have noticed a recent sharp reduction in the amount of unwanted junk mail or spam. This is because of the introduction of a spam management technique known as "Grey Listing". Some members have experienced spam reductions of 90% or more. Although Grey Listing is not the complete answer to spam (no such tool exists) it looks likely to significantly improve the effectiveness of Melb PC's virus prevention service.

Here's How it Works The Concept

The idea of Grey Listing is to work out whether a message is coming from a legitimate sender or from a spammer. If that can be done successfully, spam messages simply won't be delivered. Grey Listing performs this minor miracle by testing whether the sending server complies with Internet regulations. The test is made taking advantage of an Internet regulation which requires that if a message is temporarily rejected it must be resent. Legitimate senders use servers which comply with that regulation — spammers, who are interested only in raw speed, do not.

Grey Listing in Practice

What happens is this

• If we receive a message for you, initially we tell the other server that your mailbox is temporarily unavailable. But at the same time we record some key details about the message so that when it arrives the next time, we will know we have seen it before.
  
  
• The other server, as required by Internet regulations, resends the message (usually in about 10 minutes) and this time we accept it. We also keep a note of the message details so that any further message of the same type can be accepted immediately.
  
  
• If the message had been from a spammer it would not be sent to us again and that's one spam message you would not see.

Figure 1. Grey Listing at work on the author's mail server. The message being temporarily rejected is from Yahoo Groups and was resubmitted and accepted shortly afterwards.

Will My Mail Be Delayed?

Initially, mail from unknown or unfamiliar servers will be delayed, but our experience shows that the delay is minimal — usually about 10 minutes. And to compensate, the Grey Listing system has an automatic white listing facility. Once we know that a particular server is genuine, it is white listed for all users across the system. That means that the longer Grey Listing goes on, the less often your mail will be delayed.

By the time you read this article, most members will be finding that almost all of their mail will be free of any delay.

Safety of Mail

Firstly members do need to keep in mind that e-mail will never be a totally 'bullet proof' delivery system. There is always a non-delivery risk — however low.

Grey Listing cannot, by its nature, permanently reject genuine mail unless the sending server is in some way misconfigured or fails to comply with Internet regulations. And in that case, the sender would always be notified. This contrasts with more traditional anti-spam systems which usually have the potential to silently delete mail.

Members experiencing problems with receiving mail should log a request for assistance with http://ihelp.melbpc.org.au/.

Is it the Answer?

Is Grey Listing the answer to our spam worries? Probably not. Although the current results are very encouraging. Members should be aware that:

1. In the longer term spammers may simply adapt their techniques to Grey Listing. They certainly have proven to be a resourceful and determined bunch. But if they do they will have to accept a considerable reduction in the speed at which they can send out messages. That won't be welcome news for them. We will just wait and see.
   
   
2. Some members have already found that there are forms of unwanted mail which the Grey Listing technique cannot prevent. Other anti-spam tools such as Melb PC's SpamAssassin, or user installed filtering such as
   PopFile may still be needed. To what extent that is so will become clearer over coming months.

Nevertheless, the addition of Grey Listing represents a significant advance in the mailbox filtering available to Melb PC members. Further significant improvements are planned. We hope this is making your use of the group's
Internet service more pleasant and trouble free.

Further reading http://projects.puremagic.com/greylisting/whitepaper.html

Reprinted from the June 2006 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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