BUYING GUIDE

Cold email inboxes: what they are and how to buy them

Everything that decides whether outbound reaches an inbox happens before the first email is written. This is the part most guides skip.

Last updated 4 September 2026

What is a cold email inbox?

A cold email inbox is a secondary mailbox on a domain kept separate from your company’s main one, used only for outbound prospecting. Keeping outreach off the primary domain means a spam complaint or a burned sending reputation hits the throwaway domain instead of the address your customers and invoices depend on. Each inbox is authenticated with SPF, DKIM and DMARC so receiving servers can verify the mail is genuinely yours.

The word “inbox” is doing unusual work here. Nobody reads these mailboxes the way they read their own. They exist to send from and to receive replies, and they are bought in batches because the whole method depends on spreading volume thinly across many of them.

Cold email mailboxes and cold email inboxes: the same thing

Cold email inboxes and cold email mailboxes are the same thing, and vendors use the two words interchangeably. Microsoft’s own documentation says mailbox, most sequencers say inbox, and pricing pages in this category split roughly evenly between them. If you are comparing providers, read a quoted price per mailbox and a price per inbox as the same unit, and check what daily send rate each one is sized for rather than assuming the words signal a difference.

Why cold email needs a separate domain

Sending reputation attaches to a domain. Once enough recipients mark mail from yourcompany.com as spam, that judgement follows every message from that domain, including the invoice you send a paying customer and the password reset your product depends on. There is no partition inside a domain between marketing mail and important mail.

So outbound runs on lookalike domains bought specifically to be spent: yourcompany.co, getyourcompany.com, tryyourcompany.com. If one degrades, it is retired and replaced. The primary domain never enters the blast radius. This is also why providers sell inboxes in hundreds rather than tens, and why domain strategy is worth reading about properly before committing to a naming pattern you will be stuck with.

Cold email domain strategy covers how to choose and structure those domains.

How many cold email inboxes do you need?

Divide your target daily volume by three. Three cold emails per Microsoft 365 inbox per day is the working figure once an inbox is warmed, so three hundred emails a day needs roughly a hundred inboxes.

That number surprises people who expect to send more per mailbox, and the instinct to push harder is exactly what burns domains. Filters are not counting your total volume; they are looking at whether one mailbox behaves like a person or like a machine. A hundred inboxes each sending three emails looks like ordinary correspondence. One inbox sending three hundred looks like what it is.

• 100 emails a day: around 35 inboxes
• 300 emails a day: around 100 inboxes
• 1,000 emails a day: around 335 inboxes
• 3,000 emails a day: around 1,000 inboxes

Those figures assume warmed inboxes and Microsoft 365. Google Workspace tolerates considerably more per mailbox, which changes the arithmetic but not the principle.

Microsoft 365 vs Google Workspace for cold email

The honest answer is that it depends on where your list reads its mail, and most comparisons skip that because it is less satisfying than picking a winner.

Microsoft 365 places better into Outlook

Mail from a Microsoft tenant into a Microsoft recipient travels a shorter, more trusted path. For B2B lists, where Outlook dominates, that is the single largest variable in whether outreach lands. Microsoft also provisions in bulk more cheaply, which is why hundred-inbox orders are usually Microsoft.

Google Workspace tolerates more volume per inbox

Twenty-five to thirty emails per mailbox per day against Microsoft’s three. If your recipients are mostly on Gmail, or you want fewer mailboxes to administer, Workspace does more work per unit. It also costs more per mailbox at the scale outbound needs.

Microsoft 365 against Google Workspace for cold email works through the sending limits, alias headroom and per-inbox costs in detail.

Buying cold email inboxes vs setting them up yourself

Setting inboxes up yourself is cheaper per mailbox and more expensive in time. The work is buying domains, creating users, writing SPF, DKIM and DMARC records for each domain, waiting on DNS propagation, and then verifying that what you wrote is what resolved. For a hundred inboxes that is a week or two before the first send.

The failure mode is what makes it worth paying to avoid. Authentication errors do not announce themselves. A missing SPF include or a DMARC record with one wrong tag does not bounce; it quietly routes a batch to spam while every dashboard shows the mail as delivered. People often discover it weeks later, from a reply rate that never made sense.

If you do build it yourself, verify the result rather than trusting the setup. The free DNS records checker reads SPF, DKIM, DMARC and MX for a domain, or up to twenty-five domains at once, and tells you what actually resolved.

What correctly authenticated inboxes do and do not decide is worth seeing in numbers. Our measured results publish 1.73% bounce across 26,698 leads contacted on this infrastructure, along with the reply rates from the same eleven campaigns, including the ones that replied at zero. Bounce rate is what provisioning is accountable for. Reply rate depends on the list and the copy, which we do not supply.

Cold email inbox providers compared: pricing in 2026

Cold email infrastructure is priced two different ways, and comparing providers requires converting to the same units first. Most charge monthly per mailbox. A few sell servers. We charge monthly per domain, a flat $30 for up to 100 inboxes, so the table below reads every offer as a price per mailbox per month.

Prices are each vendor’s own published figures, read from their pricing pages on 23 August 2026. They change, and a provider who does not publish pricing at all is not included rather than estimated. One vendor is compared in detail on its own page: Cheap Inboxes against buying outright.

The comparison that matters is the price per mailbox per month once everything is in the same unit. A hundred mailboxes at $3 each is $300 a month, which is $3,600 a year. A hundred inboxes here is $30 a month, which is $360 a year, and the domain stays in your name. Whether the gap is worth it depends on what else you want from a provider. Google Workspace, pre-warmed mailboxes and managed replacement all cost more, and some senders need them.

Why the tenant matters as much as the domain

Domain isolation is well understood: cold outreach belongs on a domain separate from the one your invoices leave from. The layer above it gets less attention and is harder to see, because nothing in a mailbox tells you who else is in the tenant with you.

Microsoft’s Tenant External Recipient Rate Limit is exactly what its name says. It caps external recipients across the whole tenant rather than per mailbox, at roughly 500 times the licence count raised to the power 0.7, plus 9,500. That is a single pool. On a tenant shared between customers, every message anyone sends draws from the same ceiling you do, and you have no way to see how much of it is left.

Reputation works the same way. Complaints, spam-trap hits and throttling events attach to the tenant as well as the domain, so a customer you have never met can degrade the path your mail leaves through. When Microsoft restricts a tenant for outbound abuse, it does not exempt the well-behaved senders inside it.

We provision one tenant per domain and never place a second domain in an existing one. That is as much self-interest as generosity: a shared tenant would put our own infrastructure at the mercy of the worst sender on it. The cost is a licensed service account per domain rather than one spread across many, which is a price worth paying for a ceiling and a reputation that belong to one customer.

It is worth asking any provider this directly, because the answer is not usually published. “Isolated workspaces” and “separate domains” are not the same claim as a separate tenant. Cold email tenant isolation sets out the sending limits involved and the four questions worth putting to any provider.

What to check before you buy cold email inboxes

Most of the difference between providers is not in the marketing. These are the questions that separate them.

• Do you own the domain afterwards, or is it rented? If you stop paying, does the sending reputation you built go with you or stay with them?
• How many other customers share the Microsoft 365 tenant your mailboxes sit in? The external recipient limit is a tenant-wide ceiling, not a per-mailbox one, so on a shared tenant your daily capacity is divided with senders whose behaviour you cannot see or audit.
• Is authentication configured per domain, or copied from a template? SPF, DKIM and DMARC have to match the actual sending path to work.
• How many inboxes share a domain? A hundred mailboxes on one domain concentrates risk that spreading across several would dilute.
• Are the inboxes warmed on delivery, or is that your job? Warmup is a two-week clock either way; the question is who is running it.
• What happens when a domain burns? Replacement policy is the thing you will care about in month three.
• Does it work with your sequencer? Most connect over standard IMAP and SMTP, which nearly everything supports, but confirm rather than assume.

The deliverability side — warmup, volume ramp, copy variation and diagnosing a reply-rate drop — is a separate discipline from provisioning. The deliverability guide covers what keeps inboxes landing once you have them.

Cold email inbox FAQs