Block Lists and the Death of a Thousand Cuts

Author’s Note: Since this writing, block lists operated by SORBS have had a pair of spectacular, catastrophic failures resulting in the inadvertent and wholly spurious characterization of enormous chunks of the Internet as sources of spam, or dynamic IP space, or both. Poor infrastructure planning and operational security precluded a graceful recovery; in fact, it was nearly a week before operations returned to normal. As such, use of GFI/SORBS as a reliable source of data on production mail servers is deprecated. My colleague Steve Atkins at Word to the Wise provides an exhaustive review of the problems leading up to and exacerbating the failures, and summarizes them neatly.

If you’re sending high volumes of e-mail, sooner or later you’ll find yourself on a block list. It doesn’t matter that all of your list segments are quadruple opt-in; like death and taxes, it’s inevitable. The secret to surviving (and correcting) a listing is to be ready before it happens. Here’s what you need to know now, before you find yourself listed.

Stay Cool. No one ever got a listing removed by screaming down a phone line or threatening legal action. Don’t expect (or demand) a good customer service experience from a block list – you are not their customer.

Block Lists Don’t Block Mail. In the initial panic following the discovery of your listing, it’s easy to forget that block lists don’t actually block any mail; it’s your recipients’ mail servers that do all the blocking. The filters used by many ISPs and companies reference data from block lists, reputation scoring firms, and especially feedback from their customers to inform their filtering decisions. If significant numbers of your recipients are reporting your mail as spam, stop worrying about the listing. It’s time to take a hard look at your list hygiene, acquisition and sending practices.

Some Block Lists Matter More Than Others. The vast majority of public block lists don’t matter at all. There are plenty of web sites that offer to look up your sending IP on hundreds of lists all at once, but unless you’re listed on one of only about a half-dozen, you probably have nothing to worry about.

So which are the ones worth worrying about? Any of the lists operated by, the CBL, URIBL, CloudMark CSI, SpamCop, Barracuda Central, and sometimes SURBL and SORBS. The cast of characters changes a little from time to time, but these are usually the heavy lifters.

Different Lists Do Different Things. A listing on the Spamhaus SBL means something very different from a listing on URIBL, which is entirely different again from a listing on Spamhaus PBL. Only one of these (SBL) is a list of suspected spam sources. The URIBL lists domains that appear in spam. The PBL is a list of IP space from which unauthenticated e-mail is not supposed to be sent. Don’t assume you’ve been listed because someone thinks you’re sending spam; make sure you understand the reason for your listing before you waste time fixing a problem you don’t have.

Many Block Lists are Automated. Some block lists operate with as little human input as possible. The URIBL is a good example. It automatically adds the domains it sees in the links contained in spam, so that users of the list can block mail based on presence of those domains. The good news is that delisting is pretty straightforward – just submit a short request on their web site. But expect the listing to be reinstated automatically if it sees more spam that contains links to the offending domain.

Avoid the Death of A Thousand Cuts. The most dangerous block lists are the private, home-grown lists created and maintained by IT professionals at the companies you’re sending to.  These lists are unpublished, unqueriable, and are controlled by harried mail administrators who don’t have time to check every few weeks to see if it’s okay to delist you.

Once you land in one of these lists, the effect is very localized, but extremely difficult to reverse. Land in enough of these lists, and you’ll notice significant deliverability problems with your target niche – the death of a thousand cuts. Ironically, one of the benefits of the large, centralized block lists for senders is that it takes just one delisting to get mail unblocked across great swathes of the Internet. It’s a lot easier than contacting every domain you send to, one by one.

Block lists seem a lot less scary once you understand how they’re assembled and used. If you find yourself listed, keep calm, find out why, and gather the data together you need to fix it.

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