Also called blocking lists, blocklists are lists of IP addresses and/or domain names that are used as data to feed into various email servers and spam filters. The implication is typically that by inclusion of a particular IP address or domain name on that blocklist, that the publisher of that blocklist perceived or believed that the user or owner of that IP address or domain name was engaged in sending spam, or some other unsavory practice. Thus, listing on a blocklist means that mail from your IP or domain would be likely to be rejected by any ISP, MBP or other mail server that subscribes to that particular blocklist.

In the past, it was common for some folks to call them "blacklists," though most operators of blocklists avoided that term. Today, most folks refer to them as "blocklists." The term "blacklist" is best avoided because of racial overtones, potential for legal liability, and checkered history of a term meant simply to indicate a component of a spam filtering system.

Technical terms that typically refer to a blocklist include:

  • DNSBL (Domain Name Service Blocking List): The most common technical term for a blocklist that lists IP addresses.
  • RBL (Realtime Blackhole List): Technically refers (only) to the MAPS RBL, one of the first blocklists, but some folks use it interchangeably with DNSBL. I recommend against using this term for multiple reasons; perhaps most importantly, spam filter rejections are not "black holes."
  • RHSBL (Right Hand Side Blocking List): The most common technical term for a blocklist that lists domain names.

I run a site called DNSBL Resource, a sort of companion site to Spam Resource, where I've documented lots of information about lots of blocklists over the years. You'll find I don't post much new info over there nowadays, as the number and broad use of blocklists seems to have perhaps waned over the years. The site once got so much traffic that I made a modest amount of pocket money from advertisements -- but by now, it's basically nothing more than a "blocklist graveyard" -- tracking the many blocklists that have been shutdown over the years.

Anybody in the world can publish a blocklist and choose to put your IP addresses on it for any reason. While some blocklists do provide good indicators that a sender has list hygiene issues (sending mail generating complaints or being received by spamtrap addresses), criteria for listing can vary greatly. And keep in mind, just because somebody publishes a blocklist does not mean that finding your IP address or domain listed on that blocklist will directly translate to deliverability issues. It could relate but does not always relate -- best to investigate and ask your friendly neighborhood deliverability consultant for guidance.

As I mentioned in my history of spam filtering, blocklists were one of the first types of broadly used spam filters. But since then, ISP spam filters have taken over that first position as far as deciding what mail gets through to the inbox, in most cases. While Spamhaus and a few other blocklists are still used broadly by large ISPs, there's also now a long tail of blocklists that are only very lightly used for active spam filtering.

Want to check to see if your IP address or domain is listed on a blocklist? Here's a tool I've put together to help you with that, called KBXSCORE.

If you want to learn more about blocklists, check out RFC 5782 which covers the technical bits of running a blocklist, and RFC 6471 covers best practice recommendations relating to blocklist operation and management.

Want to learn more about deliverability terminology? If so, be sure to visit the DELIVTERMS section here on Spam Resource.

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