A blackhole list is used by some antispam vendors

A "DNS black hole list" is a form of antispam filter used by some anti spam vendors

There are many ways that vendors use to identify junk e-mail. This is one of them:

A spam DNS blackhole list, (DNS = Domain Name Server), or DNS BlackList (DNSBL for short), is a list of internet computers identified by their IP addresses that are known to have actively participated in sending spam e-mail, either deliberately or by some side effect of their mis-configuration. For example they have been hacked and had a proxy server installed, they have an open relay e-mail server, they are running an incorrectly configured proxy server, or they are running an unpatched web server that can be used to send spam.

Several organizations operate DNSBL servers that can be queried by anybody to see if a certain IP address is in their blacklist. Some servers are more restrictive than others in adding IPs to their lists and ensure they only add the worst or most easily identified offenders. Others can be overly ambitious and retain IPs in their list long after the system in question has been "fixed" or re-used by another innocent party, resulting in false positives.

K9, available via our Home Page, has the option to use a DNSBL server to aid in identifying spam by taking the IP addresses found in the e-mail headers and querying the DNSBL server specified in the List Server field when e-mail arrives. K9 does not directly associate a DNSBL hit with spam; if a hit occurs K9 will increment the count of a special keyword in its database (either the Good database if a DNSBL hit occurs on a Good scored e-mail, or in the Spam database if a DNSBL hit occurs on a Spam scored e-mail). This will help in identifying the validity of the DNSBL list in identifying spam IP addresses by letting K9 learn over time how useful the results are.

If a DNSBL hit occurs on an e-mail that has already been identified as Spam when the e-mail arrives, K9 will add the word [DNSBL] to the e-mail's Subject or header line, in addition to the [Spam] marker. The program does not add this special marker if the e-mail was identified as Good.

K9 can keep a list of IP addresses that it has already checked against the DNSBL in memory, (a cache), so that it can reduce the time needed to lookup an IP address. You can choose how long it keeps entries in its IP cache by specifying the number of days in the "Days to cache" address box.

If you have this option enabled and experience slowdowns during e-mail checking or you are on a very slow dial-up connection you may want to disable Blackhole list checking since the time required to query the remote DNSBL server over the network can affect the overall speed of K9.

The above description of a DNS blackhole list appears on Robin Keir's K9 web. More on this, and similar subjects, may be found via the following link: A copy of K9 is available via our Home Page. Just scroll down to about the middle of the page and you'll see it!
Problems with Spam in Outlook 2010? - Try Spam Reader - free anti-spam plugin for Microsoft Outlook

http://www.keir.net/k9_advancedtab.html