Illingworth lists his domain rules, 10-15 sends per inbox
Fourteen thresholds in one post, most of them familiar, now sitting next to each other.
Richard Illingworth posted his cold email infrastructure rules as a single list of fourteen lines. Keep the root domain dark. Run up to 20 subdomains off one root. Two to three inboxes per subdomain, never more. Build subdomains on a .com, never a .info. Keep the brand name first in every domain, give every inbox a real human name rather than info@, and split Google and Outlook 70/30 or 50/50.
split Google and Outlook 70/30 or 50/50
On volume he lands at 10 to 15 sends per inbox per day, with no links in the first email.
Reply rate per domain, not per inbox
The measurement half of the list is the part worth re-reading. Illingworth says to read reply rate per domain rather than per inbox, ignore a half-percent dip, and act only when a domain's reply rate drops 50%. When that happens, rebuild it on a new admin panel before buying new domains, and retire the domain after its third rebuild.
That retirement threshold is one he has made earlier as a standalone argument, and the per inbox daily ceiling is the same number he has defended before by pricing what each extra send costs an inbox's lifespan. The new thing here is not a test. It is the thresholds sitting together in one place, which is how you would actually audit a setup.
What to check against it
Run the arithmetic on your own build before you bookmark it. Twenty subdomains per root at two to three inboxes each is 40 to 60 inboxes off a single root domain, and at 10 to 15 sends a day that is a few hundred sends a day per root. Anyone sending at agency volume off a handful of roots with eight or ten inboxes per subdomain is outside every line of this list at once.
The per domain measurement rule matters more than it reads. At two to three inboxes per subdomain, a single inbox's reply rate is built on too few sends to tell you anything, so per inbox numbers will swing on noise and push you into retiring things that are fine. Rolling up to the domain gives you a sample worth reacting to, and the 50% trigger is what keeps you from reacting at all until the signal is unambiguous.
This is one operator's rule set, not a dataset. Illingworth does not attach send volumes or reply rates to the post, so treat the numbers as his operating thresholds rather than tested optima.
Know what is landing before you burn another domain
One email a day. Under three minutes. What the operators found out yesterday.