How many domains and inboxes do you need for cold email?
Inboxes needed = daily sends ÷ the per-inbox rate for your inbox type (Google 15–20, Outlook 3–10, SMTP 11–14). 150k a month takes about 770 inboxes on 59 domains.
- There's no universal "sends per inbox" number. It depends on the inbox type: Google 15–20 a day, Outlook Premium 8–10, Outlook Normal 3–5, Private SMTP 11–14.
- Inboxes per domain depends on the type too: 5 for Google and SMTP, 100 for an Outlook tenant.
- Formula: inboxes = monthly sends ÷ (sends per inbox per day × 20 sending days), then domains = inboxes ÷ inboxes per domain.
- On a 50/30/20 Outlook, SMTP and Google stack, 150,000 emails a month takes about 770 inboxes on 59 domains, plus 50% warm reserve.
"How many inboxes do I need?" is usually answered with one magic number of sends per inbox. That number doesn't exist. A Google Workspace inbox, an Outlook tenant inbox and a private SMTP inbox each have their own safe daily rate, their own number of inboxes per domain and their own warm-up time. Size them as one and you either overpay or burn domains.
The numbers below come from our 2026 Q3 inbox-types data, the rates we run client infrastructure on.
How many emails can each inbox type send per day?
| Inbox type | Cold sends per inbox per day | Inboxes per domain | Sends per domain per day | Sends per domain per month |
|---|---|---|---|---|
| Google Workspace | 15–20 | 5 | ~100 | ~2,000 |
| Outlook Premium (Azure tenant) | 8–10 | 100 | ~900 | ~18,000 |
| Outlook Normal (Azure tenant) | 3–5 | 100 | ~400 | ~8,000 |
| Private SMTP | 11–14 | 5 | ~65 | ~1,300 |
Monthly figures assume about 20 sending days. These are rates for warmed inboxes. During warm-up, cold volume starts at 2 a day and ramps by 2–3 a day on Google and 2 a day on Outlook. The full schedule is in the warm-up guide.
Google wins on deliverability. Outlook wins on volume per domain: one Outlook Premium domain does the monthly volume of about nine Google domains.
The formula
inboxes = monthly sends ÷ (sends per inbox per day × sending days)
domains = inboxes ÷ inboxes per domain (round up)
reserve = domains × 1.5 (keep 50% extra warm)Worked example, 150,000 a month on Outlook Premium only:
- 150,000 ÷ (9 × 20) = 834 inboxes.
- 834 ÷ 100 = 8.3, so 9 domains.
- Keep 50% warm in reserve: about 14 domains in total.
The same 150,000 on Google only would take about 429 inboxes on 86 domains. Same volume, ten times the domains.
How many inboxes for 10k, 50k, 150k and 300k a month?
Single provider, sending capacity only (no reserve):
| Monthly sends | Outlook Premium | Outlook Normal | Private SMTP | |
|---|---|---|---|---|
| 10,000 | 29 inboxes / 6 domains | 56 inboxes / 1 domain | 125 inboxes / 2 domains | 40 inboxes / 8 domains |
| 50,000 | 143 / 29 | 278 / 3 | 625 / 7 | 200 / 40 |
| 150,000 | 429 / 86 | 834 / 9 | 1,875 / 19 | 600 / 120 |
| 300,000 | 858 / 172 | 1,667 / 17 | 3,750 / 38 | 1,200 / 240 |
Outlook tenants come in blocks of 100 inboxes per domain, so you'll own the whole block even if you need fewer.
Why you shouldn't run one inbox type
Single-provider setups fail all at once. A provider outage, a policy change or a batch of burned domains takes out your entire sending capacity. We run a diversified stack instead:
- 50% Outlook Premium: the bulk workhorse. Fast warm-up and high volume per domain.
- 30% Private SMTP: a cheap buffer. It absorbs spikes and protects the other layers.
- 20% Google Workspace: the premium layer, for your best segments.
| Monthly sends | Outlook Premium (50%) | SMTP (30%) | Google (20%) | Total sending | With 50% warm reserve |
|---|---|---|---|---|---|
| 10,000 | 1 domain / 100 inboxes | 3 / 15 | 2 / 10 | 125 inboxes, 6 domains | ~9 domains |
| 50,000 | 2 / 200 | 12 / 60 | 6 / 30 | 290 inboxes, 20 domains | ~30 domains |
| 150,000 | 5 / 500 | 36 / 180 | 18 / 90 | 770 inboxes, 59 domains | ~89 domains |
| 300,000 | 9 / 900 | 72 / 360 | 35 / 175 | 1,435 inboxes, 116 domains | ~174 domains |
150,000 and 300,000 a month line up with Zylar's Growth and Scale plans. Zylar doesn't charge per inbox, so 770 inboxes costs the same on the sequencer side as 7. What those inboxes cost you is in what cold email really costs.
Google vs Microsoft: official limits and why cold caps sit far below them
Both providers publish daily limits. Neither limit is the number you should plan around.
Google Workspace. Google's Gmail sending limits for paid accounts are 2,000 messages per user per day, 10,000 total recipients per day and 3,000 external recipients per day. Trial accounts are limited to 500 messages per day. Google notes these limits can change without notice.
Microsoft 365. Microsoft's Exchange Online limits set a recipient rate limit of 10,000 recipients per day per mailbox and a message rate limit of 30 messages per minute on Business Basic and Standard. There's also a tenant-level external recipient limit that depends on how many licenses you hold, explained in Microsoft's tenant outbound limits post. Trial tenants are capped at 5,000 external recipients per day.
Two Microsoft details trip people up:
- Microsoft had planned a per-mailbox limit of 2,000 external recipients a day. It cancelled that plan in January 2026. You'll still see the 2,000 figure quoted in older guides. The tenant-level limit remains.
- Mail sent from the default
onmicrosoft.comdomain is limited to 100 external recipients per organization per day. Always send from your own domain.
So why send 3 to 20 a day when you're allowed 2,000? Because these are abuse ceilings, not reputation guidance. Gmail and Outlook decide inbox or spam based on how recipients react to your mail: opens, replies, deletes, spam complaints. Cold email goes to people who didn't ask for it, so every send carries some risk of a complaint. Google's sender guidelines ask senders to keep the spam rate reported in Postmaster Tools below 0.10% and never reach 0.30%. A few hundred cold sends a day from one inbox makes that hard to control. Low per-inbox volume spread across many inboxes keeps each sender looking like a person, and contains the damage when something goes wrong.
The rules for each provider are covered in Gmail, Yahoo and Microsoft bulk sender rules. If your mail lands in Outlook junk specifically, see why cold email lands in Outlook junk.
Keep reserve capacity, not just a buffer
Our rule: keep 50% of your total capacity warm as a reserve. If a provider goes down or a batch of domains burns, the volume moves to warm inboxes the same day. Otherwise you're waiting days (Outlook) to weeks (Google, SMTP) for replacements to warm up.
- Keep spare domains warming. Domains that are bought, authenticated and on warm-up, but not yet sending cold.
- Leave headroom in caps. If every inbox runs at its maximum, a paused domain's volume has nowhere to go.
- Keep the stack diversified. One provider's bad week shouldn't stop everything.
In Zylar, when a domain's placement drops below your threshold, it auto-pauses, its volume moves to healthy domains, and it goes back on warm-up until it recovers. That only works if the healthy domains have room, which is exactly what the reserve is for. For the manual version of that process, see burned domain recovery.
Know where you land before you hit sendPlacement tests per domain, daily health scores and auto-pause.How it worksA quick sizing checklist
- Write down your monthly volume including follow-ups.
- Decide your stack split, for example 50% Outlook Premium, 30% SMTP and 20% Google.
- For each layer: inboxes = sends ÷ (that type's per-inbox rate × 20).
- Domains = inboxes ÷ 5 for Google and SMTP, ÷ 100 for Outlook. Round up.
- Add 50% for the warm reserve.
- Use generic, high-quality brand domains. Aged domains (a year or older) work well right now.
Then do the setup itself: DNS, tracking domain, warm-up and monitoring. The full process is in the cold email infrastructure checklist, and you can check any domain's records with the free SPF/DKIM/DMARC checker.
FAQ
How many emails can I send per inbox per day?
It depends on the inbox type. From our 2026 Q3 data: Google Workspace 15–20 cold emails a day, Outlook Premium 8–10, Outlook Normal 3–5 and Private SMTP 11–14, once warmed up. The often-quoted single number of 30–50 a day isn't safe for any of them.
How many inboxes can I put on one domain?
Five on Google Workspace and on Private SMTP. An Outlook Azure tenant carries 100 inboxes on one domain, which is why Outlook gets so much volume per domain.
How many inboxes do I need for 150,000 emails a month?
On a 50/30/20 Outlook Premium, SMTP and Google stack, about 770 inboxes on 59 domains, plus 50% warm reserve (about 89 domains). On Outlook Premium alone, about 834 inboxes on 9 domains. On Google alone, about 429 inboxes on 86 domains.
Should I use Google or Outlook inboxes for cold email?
Both. Google delivers best, so use it for your highest-value 20%. Outlook Premium gives the most volume per domain and warms up in days, so make it the 50% workhorse. Add Private SMTP as a cheap 30% buffer. Running one provider is a single point of failure.
How much reserve capacity should I keep?
50% of your sending capacity, kept warm. Replacements take days to warm up on Outlook and weeks on Google and SMTP, so a cold reserve doesn't help when a domain burns today.
