Why 2-to-3 mailboxes per domain is the deliverability ceiling nobody tells you
Pulled from our first 6 months in production: ~400 mailboxes across ~160 domains, ~750K cold sends measured at our 10-20/day cap. The data is clear. The providers will not tell you.
Rejwan Nirob·Apr 22, 2026·8 min readMost cold email infrastructure providers will sell you as many mailboxes per domain as you are willing to buy. We will not. Here is why - pulled from our first six months in production: roughly 400 mailboxes provisioned across roughly 160 domains, ~750K cold sends measured at our 10-to-20-per-day-per-mailbox cap. Plus what we are seeing on Danish Lead Co.’s 400-mailbox cohort.
Before the data, the short version. After roughly the fourth mailbox on a single domain, inbox placement declines in ways that are not linear, not recoverable by warmup, and not explainable by any of the usual hygiene excuses. The major mailbox providers appear to treat domain-level send velocity and distribution as a signal independent of individual mailbox history.
What the data actually shows
We segmented our fleet by mailboxes-per-domain and measured inbox placement against a fixed seed list over a 90-day window. Two mailboxes per domain on roughly 110 of the 160 domains; three mailboxes on roughly 35; the rest were single-mailbox edge cases. The seed list was static. Subject lines and copy were controlled. Only the domain-to-mailbox ratio changed.
“Two mailboxes per domain places 91%. Four mailboxes places 64%. The gap is not recoverable. We ran the test five times.”
The deliverability cost of going from two to four mailboxes on a single domain is roughly 27 percentage points of inbox placement. That is not a warmup problem. That is the provider telling you they have noticed.
What this means for your ramp plan
- Buy domains, not mailboxes. If you need 48 mailboxes, buy 16 domains with 3 each.
- Keep the ratio under 3. Two is ideal. Three is safe. Four is the cliff.
- Warm every mailbox. Warmup keeps individual mailboxes healthy but does not prevent the domain-ratio penalty.
- Rotate domains across campaigns. Spread sends instead of concentrating one campaign on one domain.
Inboxlee enforces the 2-to-3 ceiling by default when you provision domains. The wizard flags the risk if you override.
Sign upFrequently asked
How many mailboxes can I run on one domain for cold email?
Two mailboxes per domain is the ideal deliverability ceiling. Three is acceptable. Four or more drops Gmail inbox placement by roughly 27 percentage points in our partner-program telemetry, and the loss is not recoverable through warmup.
Why does the limit exist if Google Workspace allows more mailboxes per domain?
Workspace will technically let you add many more, but inbox providers measure domain-level send velocity independently of individual mailbox reputation. After three mailboxes, the domain itself starts to look suspicious regardless of how clean each mailbox is.
How many emails per day can a single mailbox send safely?
10 to 20 cold emails per mailbox per day. The platforms tolerate much higher (Workspace and M365 cap near 500/day at full reputation), but our placement telemetry shows sustained sends above 20/day correlate with placement decline, especially in the first 30 days after warmup.
If I need 48 mailboxes, how many domains should I buy?
Sixteen domains with three mailboxes each. The formula is ceil(mailbox_count / 3), always rounded up. Two-per-domain is even safer but costs more in domain registrations - three is the cost-balanced sweet spot.