8/11/2004
Black Holes and other Spatial Phenomenon
ISPs simply blocking access to their mail servers to nations responsible for generating a lot of spam can be helpful for home-based end-users, but corporate email requires connectivity to all parts of the globe. Missing a significant email can have equally disastrous effects as spam itself. Enter the public Black Hole lists. A spam filter catching a piece of legitimate business mail is called a False Positive, and it is the worst possible scenario for a spam filter.

Black Hole lists are large repositories of IP addresses, email addresses and/or domains that are known to be spamming. They are typically volunteer-run and make their databases of spammer information available to the public for free, although some are professional commercial companies who sell their Black Hole service. These repositories use various reporting mechanisms ranging from human reporting to spam-trap
email boxes to determine who is sending spam, and when a spammer is identified by IP or domain or IP block, the spammer is added to the Black Hole list. Other ISPs and email providers can configure their email servers to query the Black Hole list any time a new email comes in. When a new mail arrives at the server, prior to putting it into the recipient’s mailbox, the server will examine the email, and trace its origin. Then it will ask the Black Hole list if this email came from a source that is a currently-know spammer. If the email does not originate from a source known to be spamming, it will be properly delivered into the recipient’s email box.
If the mail fails the test and is flagged as spam, the mail will not be delivered, but rather will be moved to a storage box for future examination by either the system administrator or the end-user.
Some Black Hole lists hold spammer information for a very long time, and are therefore not current.
Since individual volunteers typically run Black Hole lists, they operate with different criteria for inclusion in their list. Some list only those spammers who are well known and malicious, like the ROKSO. ROKSO lists only spammers who have been thrown off of at least three ISPs for spamming violations. Most of the people on the ROKSO list are criminals, and many have multiple fraud convictions. The fact that ROKSO is so discriminating about who qualifies as a spammer, and that ROKSO update their list every hour means that this list rarely gives false positives.
Mails flagged as spam by ROKSO are almost always trash. Other Black Hole lists are less discriminating, and require only a few people to receive and report spam from an address before they will flag that address as a distributor of junkmail. This hypersensitivity may have its place in weighted systems, however liberally labeling email as spam increases the likelihood of a False Positive, an unacceptable situation.
Other Black Hole lists use altogether different criteria. Several maintain lengthy lists of all known dynamic
(dialup, DSL and cable-modem) IP addresses.
These are addresses that should never be sending mail directly to an ISP’s mail server, but are used frequently by spammers because they are easy to obtain anonymously and disposable. Double-checking to see if an incoming email originated from a dynamic IP address is another good weight to add to a spam filter, however like the more liberal spam lists, it should not be used to flag mail as spam directly, since it is likely to produce some false positives.
Still another Black Hole methodology designed to combat spam is the Open Relay and Open Proxy lists. These lists are maintained by groups of volunteers who scan the Internet for mail and proxy servers that can be commandeered by spammers and tricked into sending spam campaigns. If a company receives a mail from a mail server that is configured as an Open Relay or an Open Proxy, it is very possible that it is a spam, but there is no way to tell for certain. This type of Black Hole list should be used along with the others above, to augment a weighted spam interception system.

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