The recent surge in Spam, specifically the Spammers use of pdf-attachments to thwart Bayesian Anti-Spam filters puts renewed emphasis on blacklists. Blacklists often referred to as DNSBL’s are in essence public databases of reported spammers IP addresses. Most mail servers and Anti-Spam solutions have built-in capabilities to query any number of these blacklists to filter out Spam.
The use of blacklists is not without controversy. Many are maintained as community projects and often without clearly published policies. In fact, there is no global standard that governs the operation of any of these lists. They are a great tool in the arsenal for fighting Spam and we happily use them, but you may not appreciate their value as much when the table turns and your company’s mail server is wrongfully listed. Getting removed can be a time consuming and frustrating process.
“If you send out an email newsletter, you may have experienced the hassle of being blacklisted from an ISP or web service. The asymmetrical nature of spam makes this particularly painful–professional spammers don’t mind being blacklisted, because they regularly switch identities. It’s the good guys (and the amateur spammers) who get hassled.”
In addition to teaching your company’s marketing department on how to run proper email campaigns that keep your company off blacklists, you should also monitor all outbound email traffic for patterns of abuse. The problems can arise from hidden sources, such as: virus infected zombie machines that send massive spam mailings unknowingly from your network, or rogue employees that intentionally run some “side-business”.
Another good practice is to pro-actively monitor the most common blacklists rather than waiting for employees to complain about email delivery problems. There are many free blacklist lookup tools available on the Internet as well as paid monitoring services.