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Planning
How many inboxes and domains do you actually need?
It is four lines of arithmetic. Below is the method, a worked example, and the one decision the number on its own will not make for you.
Want it worked out for you? Warm Inboxes already runs this as a calculator — put your target in and it returns the inboxes, domains and cost. We are not going to build a second one.
Open the calculator at Warm Inboxes ↗ Domain & Inbox Planner ↗
The four steps
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Inboxes: divide your daily target by 15
Fifteen cold emails per inbox per day is what we run and what we recommend. It is deliberately low. The published provider limits are the point at which Google or Microsoft stop you, not advice on what is safe.
1,500 ÷ 15 = 100 inboxes
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Add about 20 per cent for replacements
Inboxes go down. If you size to exactly your target, the first domain you lose puts you under it, and you are then buying replacements in a hurry, which is when people skip the checks. Hold some back.
100 + 20% = 120 inboxes
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Domains: three inboxes on each
Three is the number that keeps a failure small. When a domain stops landing, everything on it goes with it, so the question is how much you lose at once. More inboxes per domain is cheaper and concentrates the damage.
120 ÷ 3 = 40 domains
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Cost: price both shapes, not one
The same 120 inboxes cost wildly different amounts depending on whether you buy packs or a tenant, and the cheaper one is not automatically the right one. That is the next section.
40 packs of 3, or 2 tenants
The decision a calculator will not make for you
At 1,500 sends a day you need 120 inboxes either way. What changes is the shape you buy them in, and the two shapes are not the same product at different prices.
| Packs | Azure tenants | |
|---|---|---|
| What you buy | 40 packs of 3 | 2 tenants of 100 |
| Roughly per month | $720 | $100 |
| Domains involved | 40 | Far fewer |
| What one failure costs you | 3 inboxes | Up to 100 at once |
| Who it suits | Most teams | Operators who already run a tenant carefully |
A tenant is the cheaper line item by a long way, and it asks something in return: all 100 mailboxes share one reputation, so a single unverified list can damage every one of them at the same time. Under about 1,000 sends a day the saving is not worth that, and you should buy packs. Above it, buy a tenant and run it properly — verified lists only, mailboxes started in batches, bounce rate checked daily for the first fortnight.
The full risk model is on the Azure tenant page, and the two are compared directly in Azure tenant vs Microsoft 365 packs.
Then check what waiting costs you
Sizing tells you what to buy. It does not tell you whether to buy it warmed or warm it yourself, and that decision usually turns on the value of the weeks rather than the price of the mailbox. The warmup tax calculator puts a number on the 51 days.