Microsoft Forefront Server Protection Blog

The official blog of the Forefront Server Protection product team.

Forefront DNSBL… Yeah or Nay?

As you might guess, DNSBL stands for DNS Blocklist. While it’s not a new technology, the usefulness of various DNS/RBL blocklists in fighting spam is indisputably immense. Over the years I’ve heard both the success stories from folks who implemented RBLs in their Exchange server deployments, and I’ve heard some horror stories from the folks who’s IPs were maliciously or mistakenly added to RBLs and the difficulties they had working with blocklist providers to delist their IPs from the RBLs. Another contributing factor to the overall painful experience with RBLs is the fact that you need to configure them with appropriate response codes and delisting logic etc. It’s manual work and as such is very error-prone. Also, some of the blocklist providers will expose their lists for free to small customers only. For example, they will allow only a certain number of queries against the blocklist per day and if the query volume exceeds the allowed (and very small in reality) free amount they will either block the queries (firewall) or ask the customer to receive the blocklists via paid subscription. If you are going to use a free DNS blocklist, you need to make adjustments (lower expectations) regarding the quality of service. Considering these factors, some Exchange admins prefer to stay away from blocklists because they just do not want to go through the headache generally associated with maintaining multiple RBL providers’ configurations.

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