A sending domain is a consumable. That single idea decides most of what follows: you buy more than you think you need, you never send from the domain your company runs on, you spread mailboxes thinly across domains rather than stacking them, and you plan for replacement from the first day rather than treating it as a failure.
How many domains, worked from your target
The arithmetic runs one way. One mailbox safely carries eight to ten cold emails a day at steady state. Two mailboxes per domain is the safe shape, and three is the ceiling. So one domain carries sixteen to twenty a day. Divide your daily target by that number and you have your domain count; multiply the domain count by two and you have your mailbox count.
| Daily cold emails | Mailboxes | Domains | What it costs a year in domains |
|---|---|---|---|
| 100 | 10 | 5 | about $60 |
| 250 | 25 | 13 | about $156 |
| 1,000 | 100 | 50 | about $600 |
| 2,500 | 250 | 125 | about $1,500 |
| 5,000 | 500 | 250 | about $3,000 |
Domains are the cheapest line in the build, roughly twelve dollars a year each, and the one most often under-bought. Teams buy twenty domains for a thousand-a-day target, put five mailboxes on each, and discover the shape of that decision when a domain goes and takes five mailboxes with it.
Naming: coherent, mainstream, forgettable in the right way
The name has two jobs. It has to survive a filter that has seen millions of throwaway registrations, and it has to survive a person who types it into a browser. Variations of your real brand do both: getacme.com, acmehq.com, tryacme.com, acme-team.com. Prefixes and suffixes like get, try, hq, team and the plural of your own name are all fine.
Stay on mainstream extensions. A .com first, then .co and .io. Avoid the cheap extensions that bulk spam has made expensive to be near, avoid hyphens and digits where a clean alternative exists, and avoid anything that reads as a keyword rather than a company. None of this is a published rule. All of it correlates with disposable registration in the data that trained every filter you will meet.
One more thing: point each domain at a real page. A redirect to your main site is enough. A recipient who checks you should not land on a parking page carrying advertising for a competitor.
Buy early, because age is free and you cannot buy it later
A domain registered a month ago reads differently from one registered yesterday, and the difference costs nothing if you plan for it. Registrars also rate-limit bulk registration, ours at fifty a day, so a two-hundred-domain build is a four-day job at the registrar before anything else starts. Buy in batches ahead of the campaigns that need them and let the ageing run in the background.
The records, in the order to write them
Every sending domain needs one merged SPF record, a DKIM key that genuinely signs, DMARC starting at observation, and MX records so replies and bounces arrive. Two SPF records on the same host is an automatic permanent failure, and so is an include chain that crosses ten DNS lookups. The full detail, including the Microsoft DKIM target that has to be read from the tenant rather than guessed, is in the authentication guide.
Then verify each record against a public resolver rather than the registrar dashboard. The dashboard shows what you saved; the mailbox provider sees what a public resolver serves, and the gap between those two is where most silent failures live.
Mailboxes per domain, and why the number is two
The domain, not the mailbox, is the unit a provider eventually blocks. Putting five mailboxes on one domain does not give you five times the throughput for as long; it gives you five times the loss when the domain goes, and it makes the domain look denser than a real company of that size. Two is the working number. One is fine. Three is the point past which you are concentrating risk for a convenience that saves twelve dollars a year.
Use different personas on different domains as well. The same three names across twenty domains is a pattern, and pattern detection is the entire job of the system you are trying to reach.
Warm-up, and what the three weeks are actually for
A new mailbox starts at its provider's floor, a handful of messages a day, and ramps toward its cap over two to three weeks. During the ramp, warm-up traffic and campaign traffic share the same daily number, so a mailbox warming at eight a day has room for two campaign sends, not ten. The three weeks buy a sending history: a record of mail that got opened, replied to and rescued from junk, which is what a filter reads when it decides where the next message goes. Detail is in the warm-up guide.
Retiring a domain before it costs you
Read the refusals, not the reply rate. A 5.1.1 rejection means the address does not exist and says nothing about you. A 5.7.x rejection is a verdict about the sender: the receiving system looked at your domain, your address or your authentication and declined. A rising 5.7.x count on one domain is the earliest reliable signal that the domain is finished, and it appears weeks before anyone notices fewer replies.
Two rules follow. Never suppress a recipient because of a 5.7.x, because the recipient did nothing and suppressing them quietly destroys a list while the real fault stays in place. And rotate early, because each extra day of sending from a domain with provider-level refusals teaches more filters the same lesson. Keep replacements bought and aged so the rotation is a swap rather than a three-week gap.
What the whole thing costs
Per domain per year: about twelve dollars. Per mailbox per month: $8.40 for a Google Workspace seat, Microsoft's own rate for a Microsoft 365 seat, or close to nothing for the marginal mailbox on a self-hosted server that starts at $39 a month. Provisioning, if someone else does it, is a one-off; ours is five dollars a mailbox. For a fifty-domain, hundred-mailbox build the domains are about $600 a year and the seats about $840 a month.
The third column is the one that decides how people actually feel about this: the hours spent writing DNS, chasing propagation, replacing burned domains and re-authenticating tenants. At a hundred mailboxes that is a day a month. At five hundred it is a job. It is the reason the platform does the whole build behind one approval rather than handing you a checklist.
Questions, answered straight
- How many sending domains do I need?
- Divide your daily target by sixteen to twenty. That is what one domain safely carries: two mailboxes at eight to ten cold emails a day each. A thousand a day therefore needs about fifty domains, a hundred a day needs five or six. Buy a few more than the arithmetic says, because you will retire some.
- Can I send cold email from my main company domain?
- No. Reputation damage does not stay in one place. If your sending domain gets flagged, the same domain carries your invoices, password resets, support replies and calendar invites. Sending domains are consumable; your company domain is not.
- How long does a new domain need before it can send?
- Two to three weeks, and the clock starts when the mailbox starts warming, not when the domain is registered. Register early so the domain has age on it, write the DNS immediately, and let warm-up run. There is no way to buy the wait down that does not involve sending from a domain someone else has history with.
- What TLD should I use for cold email domains?
- A .com first. Then .co and .io if you must. Avoid the very cheap extensions that bulk spam has made unaffordable to be associated with, and avoid hyphens and digits where you can. None of this is a rule a filter publishes; all of it correlates with throwaway registration in the data filters were trained on.
- Should sending domains point at a website?
- Yes, even a redirect to your main site. A recipient who looks you up should find something coherent rather than a registrar parking page full of advertising. It costs nothing and it removes one of the cheapest reasons to distrust you.
- How do I know when a domain is finished?
- Watch the shape of the refusals rather than the reply rate, which lags by weeks. A rising count of 5.7.x rejections, the family that means the receiving system examined your domain and declined, is the earliest reliable signal. Rotate then, not after the reply rate falls.
- Can I reuse a retired domain later?
- Rarely worth it. Reputation attaches to the domain, decays slowly, and you cannot see what a provider remembers. A fresh domain costs about twelve dollars and three weeks. A rehabilitated one costs an unknown amount of both.
- Do I need a separate tracking domain?
- If you track opens or clicks, yes. A redirect through a shared tracking host puts another sender's reputation inside your message, and link reputation is weighed nearly as heavily as sending reputation. If you can send without tracking at all, that is better still.
Published 19 September 2026. Updated 19 September 2026. Written by the team that runs the infrastructure; numbers come from the platform's own provisioning and sending, and from the providers' published documentation at the time of writing.