Illingworth posts the subdomain rules behind 60 inboxes per domain

Richard Illingworth has published the rules behind running up to 60 cold email inboxes off a single root domain, a setup he says he tested across roughly 30,000 accounts. It extends the subdomain cost argument he made earlier into an actual build spec.

The structure

The root is one clean .com per tree, bought from Porkbun or Namecheap and forwarded through Cloudflare. It never sends, not even one campaign. Illingworth's description of its job is that it is only there so the branches have something to hang off.

Each branch is a subdomain in the form send.yourdomain. com, and 20 branches per root is what he calls the hard ceiling. Every branch holds two to three official Google or Microsoft inboxes, with SPF, DKIM, DMARC and MX set on the branch before it sends anything. The point of the structure is containment. A flagged branch gets cut off and the rest keep going.

On volume, each inbox sends 10 cold emails a day plus 20 warm-up emails a day, and warm-up and campaigns start together rather than in sequence. He says he builds to about half the ceiling in practice, roughly 10 branches per root, which leaves margin rather than packing every root full.

Where it breaks

Illingworth lists four failure modes. Somebody sends from the root, which takes down everything hanging off it. The root is a .info, which he says is fine as a domain on its own but too shaky to carry subdomains. A root is filled to all 20 branches with no margin left. And the fourth is the one worth reading twice before copying any of this.

"you only run ~30 inboxes across the whole business. this earns its keep at hundreds or thousands of accounts," he writes. He says a few of his customers send from around 10,000 inboxes this way.

Richard Illingworth

@RCIllingworth

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you only run ~30 inboxes across the whole business. this earns its keep at hundreds or thousands of accounts

For people sending, that last line is the filter. This is a cost and blast-radius structure for agencies and infrastructure resellers running inbox counts in the hundreds or thousands. At 30 inboxes, buying separate domains is not what is hurting your numbers, and the shared-root risk is real downside for savings you will not notice. The 30,000-account figure is Illingworth's own claim about his own testing, not a published dataset.