Why 2-to-3 mailboxes per domain is the deliverability ceiling nobody tells you
Why InboxLee refuses to provision a fourth mailbox on a single domain, what the domain-level signal actually is, and how to plan a ramp around it.
The InboxLee TeamMost cold email infrastructure providers will sell you as many mailboxes per domain as you are willing to buy. We will not. InboxLee refuses to provision a fourth mailbox on a single domain, and this is the reasoning behind that default.
The short version: past 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.
Why the ceiling exists
Mailbox providers score the domain, not only the mailbox. Stacking many mailboxes on one domain concentrates send velocity behind a single name, and a name that abruptly starts sending from six or eight fresh addresses reads as a domain being farmed rather than a company hiring. Spreading the same volume across more domains keeps per-domain velocity low and keeps each mailbox history legible.
“Buy domains, not mailboxes. The ratio is the lever, and warmup does not move it.”
We do not attach a placement percentage to that. InboxLee never sends on your behalf and does not currently operate fleet-wide seed tests, so we have no first-party measurement to quote - the ceiling is an operating policy drawn from how receivers score domains, not a number from an internal study.
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 ceiling. Three is acceptable. Four or more concentrates too much send velocity behind a single domain name, and the resulting placement loss is not recoverable through warmup. InboxLee refuses to provision past three on one domain.
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 that platform ceiling is a technical limit, not a deliverability-safe one. 10 to 20 is the cap InboxLee documents and recommends holding, 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.
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