The fight against spam can feel never-ending. Whether it contains potentially dangerous links to scams and malware, or just an avalanche of annoying ads, junk email is never welcome in your inbox. The good news is that anti-spam technology is always improving.

Blacklists are one of the most effective spam-filtering techniques currently available. By looking at each email address (or the domain or IP address associated with it) and checking it against known sources of spam, blacklists offer a simple way to block unwanted emails.

However, with spammers constantly switching addresses, domains, and IPs, keeping email blacklists up to date can be tough. This is where Real-time Blackhole Lists, or RBL lists, can make a huge difference.

What is an RBL?

A Real-time Blackhole List, or Domain Name System-based Blackhole List (DNSBL), is a published blacklist of IP address owners that are known sources of spam. Any IP found on an RBL can be instantly identified as spam and discarded.

An RBL blacklist typically contains computers and networks operated by active spammers as well as systems passively involved with spam. This includes internet service providers whose users are known to send spam, or machines that have been hijacked by spammers.

But unlike static email blocklists, an RBL list is highly dynamic, meaning it's updated and published on a very regular basis. That way, an RBL list subscriber constantly has the best information on all the latest known sources of spam, enabling spammers to be identified and blocked even more efficiently.

How do RBLs work?

An RBL blacklist is compiled using a variety of means. Reported sources of spam are routinely added on an ongoing basis and RBLs also use “spam traps” or “honey pots” (mailboxes specifically set up and published to attract spam) to identify new spam sources and add them to the RBL blacklist.

Once an RBL list subscriber knows which IP addresses are associated with spammers, it’s a simple case of blocking them – this usually happens during the SMTP connection phase. By subscribing to an RBL list, email recipients can check the connecting IP to see if it’s known by their RBL, then instantly drop the connection without accepting any traffic.

Find out more about spam filtering

At Fasthosts, we’ve recently enhanced the advanced spam filtering options across our entire range of email products – and this includes strengthening our RBL lists with more known sources of junk email.

Our RBLs are managed by us, so you don’t need to worry about implementing them. All you have to do is select your preferred spam filtering settings in your Control Panel. For more details on how to check an RBL list, check out our helpful guide on setting up your mailbox spam filters. You also can find out more about useful tools over on our blog.

Whether you simply need professional and personalised email hosting for your business, or the powerful functionality of Microsoft Exchange, Fasthosts offers the latest in advanced RBL spam filtering and protection from unwanted messages. It's all hosted in secure UK data centres with expert technical support available 24/7.