The Composite Blocking List (CBL) is a DNSBL that helps you block mail from exploited computers. That includes abused open proxy servers, as well as virus and trojan-infected spam spewers, the primary vector for most of the illegal spam people are receiving nowadays. By some counts, there are millions of these computers in the world, and besides spam, they’re also responsible for denial-of-service attacks, virus distribution, phishing, etc.
As the CBL website indicates, the data behind the listings is sourced from very large spamtrap-receiving domains and various email infrastructures. Their intent is to list only IP addresses that exhibit characteristics specific to open proxies, viruses, stealth spamware applications loaded on a computer without the user’s knowledge, etc. They don’t knowingly attempt to block any sort of legitimate mail. And I would characterize “legitimate” very broadly here – legitimate senders like most email service providers (and their clients) should rarely, if ever find their mail blocked by a CBL listing.
Though, on occasion, it does happen. CBL doesn’t ever list good senders intentionally. The problem is that some computers share IP addresses with others, behind a NAT (network address translation) device or firewall. Your legitimate mail could be going out to the internet over an IP address shared with an infected, spam-spewing Windows desktop. It’s fairly rare, but when it does happen, CBL makes it easy for you to address those kinds of issues, by allowing you to remove any entry from the list. This allows you to again send mail to the site that was rejecting it due to the listing. Keep in mind that if they again later see bad traffic coming from that IP, it could get listed again. That means it’s important to figure out what on your network is infected or spewing, and fix it.
I recommend use of the CBL (or one of the other lists that includes the CBL data) to filter or reject inbound mail. It helps to block some of the worst types of illegal spam out there, and the risk of blocking legitimate mail is very low.
The CBL listing data is integrated into the Spamhaus XBL (and is therefore also part of Spamhaus ZEN). If you use either of these Spamhaus DNSBLs to tag, filter or reject inbound mail, then there’s no need to utilize the CBL as well – you’re already doing so.