Oct 25, 2006

DNSBL - email block list

A DNS-based Blackhole List (DNSBL, also known as Real-time Blackhole List or RBL), is a means by which an Internet site may publish a list of IP addresses, in a format which can be easily queried by computer programs on the Internet. As the name suggests, the technology is built on top of the Internet DNS or Domain Name System. DNSBLs are chiefly used to publish lists of addresses linked to spamming. Most mail transport agent (mail server) software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists.

How Block List Providers Match Offending IP Addresses

After you set up your connection filter, when an e-mail message is sent to your organization, Exchange contacts the block list provider. The provider checks for the existence of an A (host) record in its DNS. Exchange queries for this information in a specific format. For example, if the connecting address is, and the block list provider's organization is contoso.org, then Exchange queries for the existence of the following record:

;. IN A 127. 0.0.x

which, in this case, is:

If this IP address is found on the provider's list, the provider returns a 127.0.0.x status code that indicates an offending IP address and the type of offense. All block list providers return a response code of 127.0.0.x, where x indicates the type of offense. This number varies, depending on the block list provider.

Although few people object to the principle that mail-receiving sites should be able to reject undesired mail systematically, many have voiced objections to specific DNSBLs for the following reason:

For Joe Blow to refuse emails is legal (though it's bad policy, akin to "shooting the messenger"). But if Joe and ten million friends all gang up to make a blacklist, they are exercising illegal monopoly power.


  • After a filter is defined, it will not take effect until it's applied to a smtp virtual server
  • Recipient filtering rules apply only to anonymous connections. Authenticated users and Exchange servers by pass these validations
  • Likewise, connection filters apply only to external connections. Connections made by Exchange servers within the same organization will not be affected.