Two mailboxes per sending domain, and three is the ceiling. One is also fine and is the safest shape of all. At two mailboxes sending eight to ten cold emails each, one domain safely carries sixteen to twenty a day. The reason is simple: the domain, not the mailbox, is the unit a provider eventually blocks, so five inboxes on one domain multiplies what you lose rather than what you send.
Read on 19 September 2026. The arithmetic below is what emailcampaign.ai provisions against, and the provider limits are the ones Google and Microsoft publish.
The arithmetic, in one direction
Divide your daily target by sixteen to twenty and you have your domain count. Multiply the domain count by two and you have your mailbox count. Divide the mailbox count by the provider's tenant ceiling and you have your tenants.
A hundred cold emails a day is ten mailboxes on five domains. A thousand a day is a hundred mailboxes on fifty domains, which at Microsoft is four tenants of about twenty-five users each. Five thousand a day is five hundred mailboxes across roughly two hundred and fifty domains, and at that size the infrastructure is the business rather than a side task.
Why not put five mailboxes on one domain
Because throughput is not the thing at risk. A domain that starts collecting provider-level refusals takes every mailbox on it out of service at once. At two mailboxes, losing a domain costs you sixteen to twenty sends a day and about twelve dollars to replace. At five, it costs forty to fifty sends a day and every warmed mailbox on it.
There is a second reason. A domain with five active mailboxes looks denser than a real company of that size, and pattern detection is the entire job of the system you are trying to reach. Fresh personas per domain matter for the same reason: the same three names across twenty domains is a pattern.
What the providers allow, which is a different question
Google Workspace allows two thousand messages a day from a paid account, five hundred from a trial, and three thousand external recipients a day. Exchange Online allows ten thousand recipients a day, thirty messages a minute and five hundred recipients per message. Those are abuse ceilings. Nothing in them says anything about whether your mail reaches an inbox, and approaching them from a cold mailbox is a signal in itself.
Buy the domains before you need them
A domain registered a month ago reads differently from one registered yesterday, and the difference costs nothing if you plan for it. Registrars also rate-limit bulk registration, ours at fifty a day, so a two-hundred-domain build is a four-day job at the registrar before anything else starts.
Point each sending domain at a real page, even a redirect to your main site. A recipient who looks you up should not land on a parking page carrying advertisements.
Questions, answered straight
- How many inboxes do I need to send 500 cold emails a day?
- About fifty mailboxes across twenty-five domains, at two mailboxes a domain and ten sends each. Budget three weeks of warm-up before that fleet carries full volume.
- How many inboxes for 1,000 emails a day?
- About a hundred mailboxes across fifty domains. At Microsoft that is roughly four tenants; at Google it is a hundred seats at the published seat price.
- Can I use subdomains instead of separate domains?
- You can, and it is cheaper, but a subdomain inherits and contributes to the parent domain's reputation. For cold email that is the opposite of what you want, because the entire point of a separate sending domain is that damage stays contained.
- How often should I rotate domains?
- On evidence, not on a calendar. A rising count of 5.7.x refusals on one domain is the earliest reliable signal that it is finished, and it appears weeks before the reply rate falls enough for anyone to notice.
Published 19 September 2026. Updated 19 September 2026. Written by the team that runs the infrastructure; numbers come from the platform's own provisioning and sending, and from the providers' published documentation at the time of writing.