How many inboxes and domains you need for cold email

The number of inboxes you need comes from two decisions: how many emails you want to send a day, and how many each inbox should send. The rest is division and rounding up. This guide walks through the arithmetic, explains why each inbox should send a modest amount, and covers the three things people most often leave out of the plan: follow-ups, spreading the load and spare capacity.

The arithmetic

Two divisions, each rounded up to a whole number:

1. Inboxes = emails per day ÷ emails each inbox sends per day.
2. Domains = inboxes ÷ inboxes on each domain.

Round up at both steps. You cannot buy a third of a domain, and rounding down means each inbox sends more than you planned.

If you buy inboxes by the domain, as you do with InboxesNow, it is quicker to start with domains. Each domain carries 3 inboxes and, by default, each inbox sends 30 campaign emails a day, so one domain covers 90 emails a day:

• Domains = emails per day ÷ 90, rounded up
• Inboxes = domains × 3
• Monthly cost = inboxes × $5, which is the same as domains × $15

For 900 emails a day: 900 ÷ 90 = 10 domains, 10 × 3 = 30 inboxes, and 30 × $5 = $150 a month.

If you plan to send more or less per inbox than the default, put your own number into the same formula. The arithmetic does not change, only the inputs.

Why each inbox sends a modest amount

A Google Workspace account can technically send far more than any cold email plan needs. Google's page on Gmail sending limits in Google Workspace lists a maximum of 2,000 messages a day for a paid user. That is a quota, not a target. The limit you reach first in cold email is reputation: how mailbox providers judge the mail coming from that address and its sending domain.

You will see per-inbox "safe limits" quoted in many places. Treat them as rules of thumb from other people's setups. Neither Google nor Microsoft publishes a number that makes a given volume safe. What Google does publish is direction: its email sender guidelines advise increasing sending volume slowly and sending at a consistent rate rather than in bursts. A modest, steady volume per inbox is the simplest way to follow that advice across many inboxes.

There are practical reasons too:

• Warm-up shares the inbox. Warm-up traffic keeps running alongside your campaigns. InboxesNow's default of 30 a day is campaign volume on top of the warm-up, not instead of it.
• Problems stay small. If one inbox starts having trouble, a modest volume means fewer messages are affected while you work out why.
• Replies stay manageable. Every reply comes back to the inbox that sent the email. Spreading sends across more inboxes spreads the replies too.

A worked table

Here is the arithmetic for five common daily volumes:

The table uses InboxesNow's defaults of 30 sends per inbox a day, 3 inboxes per domain and $5 per inbox a month. Daily capacity is what the rounded-up number of domains can send, which is why it can sit slightly above the volume you asked for.

The planner below runs the same arithmetic for any daily volume you type in.

Follow-ups count toward daily volume

The most common planning mistake is counting new prospects and forgetting the follow-ups. Every step of a sequence is a send from the same inbox, so it counts toward that inbox's daily volume.

Say you add 300 new prospects a day to a three-step sequence. On day one you send 300 first emails. Once the first batches reach their later steps, each day you send the first email to today's prospects, the second email to an earlier batch and the third to an earlier batch still. At that point you are sending up to 900 emails a day, not 300.

A quick rule:

• Daily volume at full speed ≈ new prospects per day × steps in the sequence

Treat the result as an upper bound. People who reply, bounce or unsubscribe drop out of the later steps, so the real number is a little lower. Plan for the upper bound anyway: in the example above, size the plan for 900 a day, not 300.

This also means volume builds gradually in the first weeks of a new sequence, as later steps fill. That suits a gradual ramp, which you want anyway.

Spread volume across domains

With 3 inboxes per domain, a larger send is spread across more domains, each carrying a small share. That is deliberate. Mailbox providers judge mail at the domain level as well as by individual address, so a domain is the unit that can run into trouble.

Spreading the load limits how much one problem can cost you:

• One domain's trouble is one domain's share. At 900 a day across 10 domains, each domain carries 90 emails. If you pause one, the others keep going while you look into it.
• You can pause and diagnose without stopping everything. Take the domain out of rotation, look at its numbers, and decide whether to rest it or replace it.
• Your main domain stays out of it. Cold email from separate sending domains keeps the domain you use for customers, invoices and everyday mail away from the risk.

Spreading volume is not a way around the rules. Google's sender guidelines ask every sender to keep the spam rate reported in Postmaster Tools below 0.1% and never to reach 0.3% or higher. Watch each domain's spam complaint rate and bounce rate, and fix the list or the message when one of them climbs. More domains do not fix a bad list.

Keep a buffer

The table gives you the minimum. Plan for a little more, for four reasons:

• Replies. Replies you send by hand go out from the same inboxes and count toward what each one sends. A campaign that works creates more of them.
• Domains that need a rest. If a domain's numbers slip, you want to pause it without dropping below your target.
• New domains warming. Any domain you add has to be warmed before it can carry full volume. With pre-warmed inboxes that time has already passed: every InboxesNow inbox is warmed for at least 21 days before it goes on sale. If you warm your own, order weeks ahead of when you need the capacity. The trade-offs are covered in pre-warmed vs warming your own inboxes.
• The ramp. Even pre-warmed inboxes should start below full volume in your own campaigns and step up over time. While you ramp, your real capacity is lower than the table says.

A simple way to size the buffer: work out the domains for your target, then add one or two spare, or a few more for a large setup. For 900 a day that means 10 domains plus one or two spare.

Putting it together

A short checklist for sizing a cold email setup:

1. Decide how many new prospects you will add a day, and how many steps your sequence has. Multiply them.
2. Decide how many emails each inbox sends a day. The InboxesNow default is 30, alongside the warm-up.
3. Divide your daily volume by the sends one domain can carry (90 by default) and round up.
4. Add one or two spare domains for replies, rests and the ramp.
5. Multiply out the inboxes and the monthly cost.
6. Once you are sending, ramp gradually and watch bounce and complaint rates domain by domain.

To see what goes into each domain before you buy, read how InboxesNow sets up its pre-warmed Google Workspace inboxes, or go straight to InboxesNow pricing to plan your monthly cost. If you are new to warm-up itself, how inbox warm-up works explains what the warm-up period does.