Infrastructure
Sequence
Money
At full volume
Warmup ramp
New inboxes do not send at full volume on day one. This is a conservative ramp — roughly what holds up on Google Workspace and Microsoft 365 without tripping rate limits or burning the domain.
| Week | Sends / inbox / day | Daily total | Week total | Stage |
|---|
Whichever provider you pick, the inboxes still need a sending tool with rotation and warmup built in. Instantly is the one most cold email teams start with.
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Set your infrastructure
Domains, inboxes per domain and daily sends per inbox give you monthly capacity.
Apply real conversion rates
Use your own numbers if you have them. The defaults are conservative B2B averages.
Read the reverse target
Enter a meeting goal and the tool tells you the fleet size and lead volume it needs.
Add the ramp
Six weeks from provisioning to full volume. Plan launch dates around it, not through it.
The three numbers people get wrong
Sends per inbox. 30 to 50 a day is the working band on Google Workspace and Microsoft 365 for cold traffic. Push past 100 and you get one good month followed by a dead domain. If a provider is selling you unlimited sending, they are selling you the burn.
Reply rate. 1 to 3% is normal for cold B2B with clean data and competent copy. Anything above 5% usually means either an unusually tight list or auto-replies being counted as replies. Filter out-of-office responses before you trust the figure, or every number downstream is inflated.
List burn. This is the constraint that actually kills campaigns. Multiply your monthly lead requirement by twelve and ask honestly whether that many qualified contacts exist in your market. Most campaigns die from running out of good leads long before they run out of inboxes — and adding inboxes to a list you cannot feed just burns through it faster.
Why the ramp is not optional
The calculator shows six weeks from provisioning to full volume, and that is deliberate. New inboxes that start at full send rate get flagged fast. The ramp is not conservatism for its own sake — it is the difference between infrastructure that lasts a year and infrastructure you replace every quarter.
Plan launches around it. If you need 20 meetings a month starting in March, the domains need registering in January. Compressing the ramp to hit a date is the single most expensive shortcut in cold email, because burned domains cannot be un-burned and the replacement cost is paid in weeks of warmup, not dollars.
Working the maths backwards
Every cold email plan is the same chain, run in reverse from the number you actually care about:
| Step | Working | Typical figure |
|---|---|---|
| Meetings wanted | the target | 20 / month |
| Positive replies needed | meetings ÷ reply-to-meeting rate | ≈ 40 (at 50%) |
| Total replies needed | positives ÷ positive share of replies | ≈ 130 (at 30%) |
| Emails to send | replies ÷ reply rate | ≈ 6,500 (at 2%) |
| Sends per day | monthly ÷ 22 working days | ≈ 295 |
| Inboxes required | daily sends ÷ per-inbox limit | ≈ 8 (at 35/day) |
| Domains required | inboxes ÷ 3 per domain | ≈ 3 |
| Fresh leads per month | emails ÷ touches per lead | ≈ 1,600 (at 4) |
The figures in the middle column are the defaults above and every one is editable. Change your reply rate from 2% to 1% and the infrastructure requirement doubles — which is the point of running it before you buy anything.
Where plans break
- List burn, almost always. Take the fresh-leads figure, multiply by twelve, and ask whether that many qualified contacts genuinely exist in your market. Most campaigns run out of good leads long before they run out of inboxes.
- Compressing the ramp. Six weeks from provisioning to full volume is not caution, it is the difference between infrastructure that lasts a year and infrastructure replaced quarterly.
- Counting auto-replies. Out-of-office responses inflate the reply rate, which deflates every downstream number. Filter them before trusting the figure.
- Solving a copy problem with volume. If the reply rate is 0.3%, more inboxes just burns the list faster. Fix the offer first.
Once the inbox count is settled, the inbox cost comparison turns it into a monthly figure, and the domains and inboxes guide covers the structure in more depth.
Frequently asked questions
How many inboxes do I actually need?
Work backwards from meetings. At 30 sends per inbox per day, 3% reply and 25% positive, roughly 60 inboxes across 20 domains produces about 58 meetings a month. Enter your own target above and the tool sizes the fleet for you. There is also a full breakdown in how many domains and inboxes you need.
Why three inboxes per domain and not ten?
Because the domain is the unit that gets burned, not the inbox. Concentrating ten inboxes on one domain means a single reputation problem takes out ten sending identities at once. Three per domain spreads that risk at a modest extra cost in domain registrations.
Are the default conversion rates realistic?
They are deliberately mid-range for cold B2B with decent data and copy. A sharp offer to a tight list beats them comfortably; a broad list with generic copy will not come close. Replace them with your own numbers as soon as you have a few thousand sends of real data.
Does the cost figure include lead data?
The tools and data field is where that goes. Verified B2B data is a real recurring cost at volume — feeding 13,000 fresh leads a month is not a trivial line item, and it is usually larger than the infrastructure cost people focus on.
How many emails can I send per inbox per day?
30 to 50 is the working band on Google Workspace and Microsoft 365 for cold traffic, and closer to 30 is the safer end. Past 100 you typically get one good month followed by a dead domain. Any provider selling unlimited sending is selling you the burn — the limit is set by filtering behaviour, not by the mailbox.
How many inboxes do I need for X meetings a month?
Work backwards: meetings → positive replies → total replies → emails sent → daily sends → inboxes. For 20 meetings a month at a 2% reply rate you land around 8 inboxes across 3 domains. Halve the reply rate and it doubles — which is why the assumptions matter more than the arithmetic.
How many inboxes should I put on one domain?
Around three. It concentrates enough sending to be worth the domain's setup cost while keeping the blast radius small — when a domain gets burned you lose three inboxes rather than ten. Ten inboxes on one domain is a cost optimisation that becomes very expensive the first time that domain is flagged.
How long before a new sending fleet is at full volume?
Roughly six weeks from provisioning: two weeks with the domain registered but silent, then a warmup ramp, then real campaign volume. Plan launches around it — if you need meetings landing in March, the domains want registering in January. Compressing this is the most expensive shortcut in cold email.
What reply rate should I plan for?
1 to 3% for cold B2B with clean data and competent copy. Above 5% usually means either an unusually tight list or auto-replies being counted as replies. Plan with the low end and treat anything better as upside, because every downstream number — leads, inboxes, cost — scales off this one figure.
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What to run next
The checks that most often follow this one.
More in this category
Guides that go deeper
When the tools tell you something is wrong
The diagnostics here are free and always will be. When the fix is bigger than a DNS record, this is the work I do.
Deliverability rescue
Mail landing in spam, replies gone quiet, or a domain suddenly blocked. I find the actual cause rather than guessing, and fix it.
- Authentication and alignment failures
- Blocklist delistings and reputation repair
- Gateway and filter-level blocks
- A written report on what broke and why
Email & sending infrastructure
Sending domains, inboxes, authentication and warmup, built to survive volume instead of burning down in a month.
- Domain and inbox fleets at any scale
- SPF, DKIM, DMARC and tracking domains
- Google Workspace and Microsoft 365 inboxes
- Handover documentation you actually own
Domain, DNS & migration
Changing registrar, mail provider or host without a day of downtime or a week of mail silently failing.
- Registrar and nameserver moves
- Workspace and Microsoft 365 migrations
- MX, SSL and subdomain cutover
- Staged rollout with rollback at every step
Monitoring & retainer
Infrastructure drifts. Records get edited, certificates expire, domains get listed. Ongoing eyes on the fleet.
- Scheduled checks across every domain
- Alerts before your clients notice
- Monthly reporting
- Priority response when something breaks
Start with a call
Bring a domain and the symptom. I will tell you what is actually wrong and whether you need me at all — plenty of people leave that call able to fix it themselves.
Thirty minutes, no pitch
We will run the checks together on your actual domains, and you will leave knowing what is broken, what it takes to fix, and what it should cost. If that is a job you can do in-house, I will say so.