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43.3% of our live sending domains are listed on SURBL. We measured what that costs, two independent ways, and the answer is not what the listing rate suggests.
2,817
Sending domains tested, every one that sent in the window
43.3%
Listed on SURBL (1,220 domains)
1.14% → 1.16%
Reply rate before and after listing. Flat
0.025%
Of all sends lost to blocklist bounces
A SURBL listing did not change reply rate or booked conversations. It did raise bounce rate, by about half. Those two facts sit together more comfortably than they look: a listing costs you a slice of delivery volume, and the slice is small enough that it never reaches the outcome anybody is paid on.
Where the domains came from mattered far more than any of it. Infrastructure we build and run ourselves came back 26.9% listed. Bought-in stock came back 63.0% listed — 2.3× worse for the same job.
Clients running campaign reviews kept asking the same question: bounces had crept up on particular domains, and was a blocklist the reason? It is a fair question and we could not answer it, because we were reading blocklist status out of bounce messages.
That turns out to be a bad instrument. Only about one listed domain in five ever produces a bounce that names the listing. Four in five are invisible to anyone watching bounces, which means every previous answer we had given on this subject was built on a fifth of the picture.
There is a second reason the question is awkward to answer from the outside. SURBL's listing criteria reach patterns of domain registration and usage, not only message content, so infrastructure built for cold email attracts listings at rates that would look alarming on a corporate mail server. A high listing rate is therefore expected. Whether it matters is a different question, and one nobody in this industry had published a number for.
So we stopped inferring and tested every domain directly.
| Population | All 2,817 domains that sent during the window. Not a sample. |
| Test | Direct DNS query per domain against multi.surbl.org |
| Why not bounce data | Only ~1 listed domain in 5 ever produces a bounce that reveals the listing |
| Validation | 5 domains known to be listed all returned positive; 3 known-clean controls all returned clean |
| Query failures | None. Every domain returned a definite answer |
| Platforms | Two sending platforms, pooled — the question is about domains, not tooling |
Estates are lettered rather than named. A and B within a provider are separate pools of infrastructure, not a split by any property we tested.
| Estate | Domains | Listed | Sends | Bounce share |
|---|---|---|---|---|
| Azure, estate AOurs | 524 | 9.5%(50) | 1,118,182 | 0.0145% |
| Azure, estate BOurs | 499 | 16.4%(82) | 912,540 | 0.0187% |
| Google, estate AOurs | 415 | 54.5%(226) | 53,829 | 0.0093% |
| Google, estate BOurs | 99 | 55.6%(55) | 29,862 | 0.0536% |
| Google, bought-in reseller stockBought in | 1,082 | 56.6%(612) | 493,428 | 0.0764% |
| Third-party SMTPBought in | 198 | 98.5%(195) | 268,218 | 0.0000% |
| All domains | 2,817 | 43.3% | 2,876,059 | 0.0254% |
Listing rate on its own is a misleading number. Our Google estate A is 54.5% listed and has the lowest real impact of anything in the table at 0.0093% of sends, because each of those domains carries only about 130 sends a month. A listed domain that barely sends costs almost nothing.
Bought-in reseller stock is the worst performer on impact, not just on listings. It accounts for 0.0764% of its own sends in blocklist bounces, about 4.7× the rate of our own Azure estates, which carry 2,030,722 of the 2,876,059 total sends at 0.0164%.
One row is a measurement problem rather than a result. Third-party SMTP came back 98.5% listed — 195 of 198 domains — on roughly 1,355 sends per domain per month, and reported exactly zero blocklist bounces. Zero is not a good score here. It means that channel does not surface blocklist rejections in a form anyone downstream can read, so its true cost is unmeasured rather than low. That gap is the reason we no longer rely on bounce messages anywhere.
| Source | Domains | Listed | Bounce share |
|---|---|---|---|
| Built and run by us | 1,537 | 26.9% | 0.0167% |
| Bought-in stock | 1,280 | 63.0% | 0.0495% |
This is the finding with the clearest practical consequence, and it is the one we did not set out to test. Domains we register, configure and warm ourselves came back 2.3× cleaner than domains bought ready-made. Same blocklist, same window, same sweep. If you are buying inbox capacity from a reseller, this is the number worth asking them for.
Run two ways on purpose. Each has a weakness the other does not share, so agreement between them is worth more than either alone.
Test 1 · Same domains, before vs after their first listing
106 domains with at least 200 contacts on each side of the listing date. Each domain is its own control.
| Metric | Before | After | Change |
|---|---|---|---|
| Reply rate | 1.14% | 1.16% | flat |
| Bounce rate | 1.41% | 2.16% | +53% |
Test 2 · Listed vs clean domains over the same period
Measured on sends needed per interested reply, which is the number a campaign is actually judged on.
2,733
sends per interested reply, listed domains
2,687
sends per interested reply, clean domains
1.7%
difference — inside normal campaign variance
Both tests say the same thing: a listing does not move replies or booked conversations. Bounce rate does rise, so deliverability is genuinely affected — this is not a claim that listings are harmless. It is a claim about where the harm stops.
A SURBL listing costs you send volume. It does not appear to cost you results.
Every cause below is measured the same way over the same window: delivery lost as a share of all sends.
Invalid addresses in prospect lists cost roughly 16× what every SURBL listing in the fleet costs put together. Recipient-side policy blocks cost about 10×.
Over the window, blocklist bounces were 731 out of 2,876,059 sends, and 1.79% of all bounces from any cause. If you are spending time on SURBL before you have spent it on list verification, you are optimising the third-largest problem first.
Detection moved off bounce messages
Bounce-based detection saw about one listing in five. The DNS sweep is now scheduled across the full estate, so listing status is known rather than inferred.
Nothing was retired wholesale
233 listed domains carrying 3,173 mailboxes were still in production and producing around 155 interested replies a month. Retiring working infrastructure over a listing that does not affect replies would have cost clients real conversations to fix a number on a dashboard.
Sourcing weighting changed
Bought-in stock listing 2.3× higher than our own is the finding with the longest tail. It is a reason to build rather than buy, and we have weighted new capacity accordingly.
The sweep behind this study is the same query our free SURBL checker runs. You can point it at one domain or paste in a whole CSV, and it costs nothing and needs no account.
If your listing rate looks like our bought-in column rather than our own, the problem is where your domains came from, not what you are sending.
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