Cold email infrastructure: how many domains and mailboxes you need

No mailbox provider publishes a number of domains or mailboxes for cold email. The counts people quote come from vendors, and they range from 10 to 50 cold emails per mailbox per day, which means 20 to 100 mailboxes on roughly 2 to 17 domains for every 1,000 emails you send daily.

What Google, Yahoo and Microsoft do publish is how mail must be authenticated, how many complaints are tolerated, and how much one account may send. This article separates those rules from the conventions and shows what the result costs.

One point first. Nothing here is a way around provider rules. Gmail's guidelines say "Don't send messages to people who didn't sign up to get messages from you", and the Google Workspace Acceptable Use Policy (last modified 13 October 2025) prohibits using the service "to generate, distribute, publish or facilitate unsolicited mass email". Any outreach programme has to live inside those terms.

Google publishes the requirements senders must meet to deliver to personal Gmail accounts. Screenshot of support.google.com, taken on 7 October 2026.

What the providers actually requireCopy Anchor Link

Google, Yahoo and Microsoft all require authenticated mail, and Gmail and Yahoo both set the complaint ceiling at 0.3%. The details differ by provider.

Microsoft announced stricter authentication requirements for high-volume senders to Outlook.com. Screenshot of substrate.office.com, taken on 7 October 2026.

For the full rule-by-rule compliance check - which of these rules a tool can actually verify, and which three (live complaint rate, content scoring, list hygiene) no tool can - see what Google and Yahoo require from bulk senders, and how to prove you meet it.

Four details in those pages matter more than the headline numbers.

• The Gmail complaint rule has two levels. Google's FAQ says senders should keep the spam rate below 0.1% and prevent it from ever reaching 0.3% or higher. A sender at 0.3% or above is not eligible for mitigation until the rate stays below 0.3% for seven consecutive days. Since November 2025, Gmail has been "ramping up its enforcement on non-compliant traffic" with temporary and permanent rejections.
• Yahoo measures against inbox mail. "Spam rate is calculated in our system based on mail delivered to the inbox", so mail already going to spam does not dilute the rate.
• The Gmail rules cover personal accounts. Google states that the guidelines do not apply to messages sent to Google Workspace accounts. Most business-to-business outreach lands in Workspace or Microsoft 365 mailboxes, where no numeric threshold is published. That is a lack of published numbers, not a lack of filtering.
• Microsoft's wording is still junk first. The Outlook.com Postmaster page says non-compliant messages go to the junk folder and that "Shortly we will reject the messages until the DNS records are corrected". A Microsoft-published complaint percentage: not found.

As plain arithmetic, on 1,000 messages delivered to Gmail inboxes, 0.1% is one spam report and 0.3% is three.

What one mailbox is allowed to sendCopy Anchor Link

The platform ceilings are far above anything a cold email vendor recommends, and they exist to stop bulk mail, not to enable it. They are the hard limits a licence comes with.

Google limits how many messages a Workspace user can send per day. Screenshot of knowledge.workspace.google.com, taken on 7 October 2026.

Microsoft is explicit about intent. The recipient limit exists "to discourage the delivery of unsolicited bulk messages", and the same page tells customers with legitimate bulk commercial email to use providers that specialise in it. A Google user who exceeds a limit cannot send new messages for up to 24 hours.

How many mailboxes and domains the vendor guidance impliesCopy Anchor Link

For 1,000 cold emails a day, published vendor guidance implies anywhere from 20 to 100 mailboxes. The spread is a factor of five because the vendors disagree on the per-mailbox number, and none of these figures comes from Google or Microsoft.

The last two columns are our arithmetic from each vendor's own numbers, rounded up. To size a different volume, divide your daily sends by the per-mailbox figure, then divide the mailbox count by the mailboxes per domain.

Two cautions about this table.

First, follow-ups count. Maildoso states its cap includes follow-ups, so a three-step sequence reaches about a third as many new people per mailbox as the headline number suggests.

Second, these are operating conventions from companies that sell mailboxes or sending software, with no published evidence behind them. Treat them as a starting range.

What spreading volume does and does not doCopy Anchor Link

Spreading volume across domains limits how much damage one bad campaign does, and it does not exempt anyone from the sender rules.

Gmail counts the bulk threshold per primary domain, with subdomains counted together, and bulk sender status is permanent once assigned. A domain that stays under 5,000 messages a day to personal Gmail accounts is not a bulk sender by that definition. But the requirements for all senders still apply at any volume: SPF or DKIM, and a spam rate below 0.3%. Yahoo words its all-sender rules the same way.

So more domains do not buy more tolerance for complaints. Each domain has its own rate, and a small domain reaches 0.3% with very few reports.

The acceptable-use terms also sit above the arithmetic. Yahoo asks senders to verify they only send mail to users who specifically requested it, and Google's policy bans unsolicited mass email on Workspace whatever the daily count.

In the United States the law adds its own floor. The FTC says CAN-SPAM "makes no exception for business-to-business email", opt-outs must be honoured within 10 business days, and each email in violation is subject to penalties of up to $53,088. Check the law where your recipients are.

The legitimate reasons to run several domains are separation and containment: keeping outreach off the domain your invoices and password resets come from, and keeping one client's reputation apart from another's.

What vendors charge per mailboxCopy Anchor Link

List prices in October 2026 run from $0.49 to $4 per mailbox per month, and the type of mailbox explains most of the gap. Vendor-run SMTP mailboxes are cheapest; real Google Workspace and Microsoft 365 mailboxes cluster between $2.50 and $3.50.

Prices are in US dollars as shown on each vendor's own page on 7 October 2026, except InboxKit and Premium Inboxes, re-checked on 11 October 2026. Billing periods differ: Mailforge and Infraforge show lower effective rates on yearly billing, Premium Inboxes does not print a billing period beside its per-inbox price, and InboxKit separately lists an Azure tenant option at $30 per tenant per month for up to 100 mailboxes. Read the billing terms too. Mailforge charges for capacity, not use: "If you purchase 10 slots but only create 5 mailboxes, you are still charged for all 10 slots". Premium Inboxes' guarantee that a month is refunded if infrastructure stops sending for more than 48 hours is stated only for its $7,500 a month managed Email Control Centre service, not the per-inbox tiers shown here.

A worked example from those list prices: 34 mailbox slots at Mailforge's $3 a month and 12 to 17 .com domains at $14 a year come to $102 a month plus $168 to $238 a year. That covers the mailboxes and names only. Sending software, warmup tools, list data and verification are separate costs.

Why this buyer needs domains and licences in bulk through an APICopy Anchor Link

An agency or outreach platform buys this infrastructure once per client, so the counts multiply past what anyone wants to click through. Using the Mailforge convention above, ten clients each sending 1,000 emails a day need about 340 mailboxes on 120 to 170 domains.

Each unit carries repeated work, and each client has to be billed for exactly what was provisioned:

• Registration. Every domain has to be checked, bought and renewed on time. A lapsed domain takes its mailboxes down with it.
• Authentication. Every domain needs SPF, DKIM and DMARC records before it sends, because the bulk rules require all three and the all-sender rules require at least one. Our free email authentication report reads all three for a domain and says what to fix first.
• Licensing. Seats go up and down as clients start and stop campaigns, and a Microsoft tenant's external sending cap depends on its licence count.
• New-tenant limits. Google's 500 a day trial limit applies to each new trial account and has to be planned around.

That is why several vendors in the table expose an API: InboxKit includes one with every mailbox, Infraforge includes one with a subscription, and Zapmail offers one on its Pro plan. When orders are placed by a script, retries and status updates need care too; see idempotency keys and webhooks for reseller APIs.

If you plan to resell this infrastructure under your own name, we compare the two programmes in Google Workspace vs Microsoft 365 for resellers, and the registrar side in the domain reseller API comparison.

Where ResellerHQ fits, and where it does notCopy Anchor Link

ResellerHQ is a white-label reseller API for domains, mailbox licences and phone numbers, and for cold email infrastructure it is not production-ready today. Here is exactly what exists.

The API can check up to 100 domain names in one call and register them one request each, and it can buy Google Workspace or Microsoft 365 seats for a domain, 1 to 300 seats per licence, with the seat count changed later. Purchases draw on one prepaid wallet and accept an idempotency key so a retry does not buy twice.

The limits are significant:

• Domains and mailbox licences run in the sandbox only. On the live side those routes answer 422 not_available, and sandbox orders are not registered or provisioned with anyone.
• ResellerHQ sells licences, not mailboxes. It does not create mailboxes, warm them up or send mail, and the mailboxes stay with Google or Microsoft under their terms.
• DNS record management is not part of the API yet, so SPF, DKIM and DMARC cannot be set through it.
• Licence renewal is automatic, but seat changes are not pro-rated. A worker charges the wallet each month a licence's paid period ends and rolls it forward; added seats are billed a full month each at the licence's price, and removed seats are not refunded for the time left.
• There is no way to assign a seat to a named user. A licence carries a seat count, not a per-seat owner.
• There is no published price list, so it cannot be placed in the price table above.

If you need live mailboxes now, evaluate the vendors in that table. If you are building a reselling product and want to test the order flow in a sandbox first, the docs describe every endpoint and signup creates a sandbox account.