· 5 min read · infrastructure
The question sounds like it should have a one-word answer, and the internet is full of confident ones: one mailbox, fifty mailboxes, a domain per hundred leads. The honest answer depends on three numbers you already know, and once you write them down the arithmetic is short.
This article works through it for the campaign we run: 2,500 leads, a four-step sequence, and four weeks of sending at a volume inbox providers consider ordinary.
The three numbers that decide it
How many emails the campaign sends. Not how many leads, how many emails. Every lead receives up to four.
How many emails one mailbox can safely send per day. For a warmed mailbox doing cold outreach, we use 30. That is a volume ordinary people send every day, and it leaves headroom below the 40 a day a mailbox reaches at the end of warmup. We explain where the ceiling comes from in email warmup explained.
How many days you want the campaign to take. Faster means more mailboxes; more mailboxes means more to set up, warm and watch. Four weeks is long enough to stay ordinary and short enough to learn something within a month.
The arithmetic
Start with the total. 2,500 leads, four steps each, gives a ceiling of 10,000 emails. In practice it is a little lower, because people who reply stop receiving follow-ups, but plan for the ceiling.
Spread over 28 days, 10,000 emails is about 360 a day. The mix changes as the campaign runs: on the first days nothing is due yet, so the volume is all first emails; once the sequence is in full swing, each day carries roughly 90 first emails and 90 of each follow-up. Either way, the number your mailboxes have to absorb is about 360 a day.
| Mailboxes | Safe daily volume at 30 each | Emails over 28 days |
|---|---|---|
| 4 | 120 | 3,360 |
| 8 | 240 | 6,720 |
| 12 | 360 | 10,080 |
| 16 | 480 | 13,440 |
Twelve is the smallest number of mailboxes that sends 10,000 emails in four weeks without asking any of them for more than 30 a day. That is why the Launch package uses twelve.
Why not fewer mailboxes?
Four mailboxes at 30 a day would need almost three months for the same campaign, which is too slow to learn anything. Pushing four mailboxes to 90 a day solves the speed problem by creating a worse one: a sending pattern no real person has, on mailboxes that are each carrying a quarter of your campaign. If one is flagged, the others look the same and tend to follow.
How many domains?
Mailboxes live on domains, and reputation attaches to both. Putting all twelve mailboxes on one domain means a problem on one mailbox can drag down the rest. Putting each on its own domain multiplies the setup work for little gain.
We use four lookalike domains with three mailboxes each. A problem on one domain affects at most a quarter of the campaign, and the other three keep sending while the first recovers. Every domain forwards to your main site, and none is ever your primary domain, for the reasons in our 2026 deliverability guide.
Putting it together
- 2,500 leads × 4 steps = up to 10,000 emails
- About 360 emails a day across 28 days
- 30 emails per mailbox per day keeps every address ordinary
- Twelve mailboxes cover the volume with no single point of failure
- Four lookalike domains, three mailboxes each, split the risk
- Each mailbox warmed for 14 days before the first send
What it costs to run
The setup is mostly time, but there are running costs too. Each lookalike domain is a small annual registration fee. Each mailbox is a user licence on Google Workspace or Microsoft 365, billed monthly per user, so twelve mailboxes means twelve licences for as long as you keep sending. Warmup is usually a monthly subscription per mailbox if you run it yourself.
None of these is large on its own, and together they are a small fraction of what one booked call is usually worth. The point of listing them is that they are recurring: when a campaign ends, decide whether you will send again within a month or two. If you will, keep the mailboxes warm. If you will not, cancel the licences and keep the domains, which hold their reputation better than mailboxes do when idle.
Running twelve mailboxes day to day
Twelve mailboxes are more than anyone wants to check by hand, and you should not have to. Replies need to reach one inbox you actually read, which is why every mailbox in our setup forwards to the reply address you choose. Health checks, meaning bounces, spam signals and whether any mailbox has been rate-limited, are worth doing daily during a campaign, because a problem caught on day two costs a day and a problem caught on day ten costs a week of sends.
If one mailbox does slip, rest it: pause its cold sends, keep it in warmup, and let the other eleven carry the volume for a few days. That flexibility is the practical reason for having twelve.
When you need more
The same arithmetic scales. Twice the leads at the same pace needs twice the mailboxes and domains; a slower pace needs fewer. What does not scale is the shortcut of sending more from each mailbox. Volume per mailbox is the number to protect, and everything else follows from it.
If you would rather not register, authenticate and warm sixteen things before your first campaign, that is exactly what domain and mailbox setup covers: four domains and twelve mailboxes, registered in your name, with every record and login handed to you at the end.