We treat a Spamhaus listing on a sending domain as fatal, and we treat a SURBL listing as something to watch. The two lists sound alike, but they cost us different amounts, so we handle them differently. This post explains the rule and why we landed on it.
Is a Spamhaus or SURBL listing worse for a sending domain?
A Spamhaus Domain Block List listing is worse, and it's the one we won't send through. If a domain shows up there, we take its mailboxes out of every campaign and replace the domain. We don't try to wait it out or clean it up while it keeps sending.
A SURBL listing gets a different response. We still see it, we still log it, and we keep sending from the domain. Our rule is that SURBL alone is never a reason to bench or swap a domain.
Our research page on sending domain blacklists covers 647 domains. About 11% were ever on Spamhaus, while 33% were ever on SURBL. Of the domains that landed on both lists, SURBL came first on 45 of 45. That order is part of why we read a SURBL listing as an early warning and not as a verdict.
Why don't we stop sending when a domain is on SURBL?
We learned this the practical way. One program's health report showed every one of its 21 sending domains on SURBL's abuse list, and a direct lookup confirmed it. If we had treated that as a stop, all sending for that program would have halted.
A rule that shuts down a whole program over one list is too blunt to be useful. So we decided SURBL is a signal we track and Spamhaus is the one that decides.
We do watch for one pattern. In one SURBL-listed pool, inbox placement was 100% at Google and 87.5% at Microsoft. A blended score would have hidden that gap, so we look at the two providers separately.
How do we check a domain against Spamhaus?
We run a direct DNS lookup against the Spamhaus domain list from our own resolver. The command is dig +short followed by the domain, then .dbl.spamhaus.org, asking for an A record. A listed domain returns an answer such as 127.0.1.2, and an unlisted one returns nothing.
dig +short yourdomain.com.dbl.spamhaus.org A # control: this one is always listed, so it must return an answer dig +short dbltest.com.dbl.spamhaus.org A
Public resolvers like 1.1.1.1 give no answer at all, so a clean result from one of them tells us nothing. To be sure the lookup works, we also check dbltest.com, which is always listed. If the control comes back empty, the check is broken and we don't trust the other results.
We run this check on every domain before a launch and before we load it with mailboxes. Our daily health check also pulls blacklist status for all of our sending domains.
What do we do the day a domain is listed?
We bench it the same day and swap in a replacement. We don't wait for a replacement to be bought first.
The swap works because of how we rotate domains. For each client the pool is split into three batches, and two of the three are active at any time. The third rests so its reputation can recover.
When a domain burns, our swap moves it into the resting batch and pulls a replacement from that batch into the active one. That keeps sending capacity the same. It also skips any reserve that is itself listed, so we never swap one bad domain for another.
The burned domain then sits for about five weeks before the normal rotation would bring it back. That gives us time to buy and warm a fresh domain without sending from a bad one in the meantime.
Do we swap on anything besides Spamhaus?
We also swap when inbox placement hits a hard floor, such as 50 out of 100 on a seed test, even with no blacklist reading at all. We have benched a domain on that reading alone.
The reasoning is the same in both cases. We swap when the evidence says mail is not reaching inboxes, and we leave domains alone when a list merely mentions them. Our free domain check reads a domain's mail records.
Common questions
Can a domain keep sending after a Spamhaus listing?
We don't send through a domain on the Spamhaus Domain Block List. We take its mailboxes out of every campaign the same day and swap in a replacement domain.
Does a SURBL listing hurt cold email deliverability?
We treat a SURBL listing as an early warning and keep sending. In one SURBL-listed pool, inbox placement was 100% at Google and 87.5% at Microsoft, so we watch the two providers separately.
How can we check if a domain is on the Spamhaus blocklist?
We run a DNS lookup of the domain followed by .dbl.spamhaus.org from our own resolver. A listed domain returns an answer such as 127.0.1.2, and public resolvers like 1.1.1.1 return nothing at all.
How common are blocklist listings on sending domains?
Across the 647 sending domains in our records, about 11% were ever on Spamhaus and 33% were ever on SURBL. On the 45 domains that landed on both lists, SURBL came first every time.