Cold Email Infrastructure: Domains, Inboxes and Daily Limits
Cold email infrastructure is the sending setup behind outbound email: separate domains that protect a company's main domain, two or three inboxes on each, SPF, DKIM and DMARC authentication, and two to four weeks of warmup before the first campaign. Each inbox sends a small daily volume, about 30 cold emails in common practice, so a company grows volume by adding inboxes rather than pushing one harder.
Picture the bouncer at a busy club. He checks your ID, and he lets in a regular before a stranger. A stranger who shows up with forty friends waits on the curb. When one of a group starts a fight, the whole group stays out next weekend, and the bouncer remembers their faces.
Gmail and Outlook run the same door for every email you send. A founder who buys a list and sends 500 cold emails from the company domain on day one looks like that stranger with forty friends. The mail lands in spam, and the damage can follow the company's ordinary mail too.
Two questions decide whether cold email reaches the inbox. Will the provider let your mail through the door, and how much can you send before it stops?
Cold email infrastructure parts
The bouncer's checks map onto five parts of the setup. Each one answers a different reason a provider might turn mail away.
- Sending domains: lookalike domains, such as a "get" or "try" version of your name, that send the cold mail and redirect visitors to your real site. A bad week then lands on them rather than on the domain your invoices come from.
- Inboxes: two to four on each sending domain, each with a real person's name and signature.
- Authentication: SPF, DKIM and DMARC records in each domain's DNS, which prove the mail really comes from you. They're the ID check.
- Warmup: a few weeks of low, rising volume that gives a new inbox a sending history before it carries a campaign.
- Monitoring: bounce and spam rates, watched weekly, so you see trouble before the providers act on it.
What Gmail and Outlook require
Google and Microsoft publish their own rules for two of those parts, authentication and spam rate. They're in Google's email sender guidelines and Microsoft's rules for high-volume Outlook senders.
| Rule | Gmail | Outlook |
|---|---|---|
| Every sender | SPF or DKIM, and spam rate under 0.3% | No published rule |
| Bulk threshold | 5,000 a day to Gmail users | 5,000 a day to Outlook users |
| Bulk senders | SPF, DKIM and DMARC, aligned | SPF, DKIM and DMARC, aligned |
| Enforcement | Since February 2024 | Since May 2025 |
A small cold campaign stays far below 5,000 a day. Set up all three records anyway, since a DMARC policy of "none" satisfies both providers and costs nothing to add. The spam rate matters more, because it's the rule cold email breaks first. Google asks senders to stay under 0.1% and never reach 0.3%, which works out to one complaint per thousand emails.
Daily limits per inbox
Neither provider publishes a safe daily volume for cold email, so the limit comes from practice. Common practice holds each inbox to about 30 cold emails a day, with 50 as the ceiling. An inbox pushed past that tends to get flagged, and a flagged inbox drags its domain down with it.
Size the setup from the list backward:
- Divide the prospects you want to reach by the working days you have.
- Multiply by the number of emails in your sequence to get daily sends.
- Divide daily sends by 30 and round up to get inboxes.
- Divide inboxes by three and round up to get domains.
Take a hypothetical startup that wants to reach 600 prospects a month with a four-email sequence. That's 28 new prospects a working day and 112 sends a day, so it needs four inboxes. Two domains with three inboxes each give it six, with room to spare.
The ratio scales. As Head of Marketing & Sales at Startup Science, I ran cold email at about 29,700 sends a month across 135 inboxes and 45 domains, three inboxes to a domain. Grow volume by adding domains, never by pushing an inbox harder.
Warmup and monitoring
A new inbox is the stranger at the door, so it needs a history before it carries a campaign. Warmup tools trade small volumes of mail between inboxes that open and reply to each other. Give that two to four weeks, and start real sends at low volume in the third week. Keep warmup running after launch, too.
Once campaigns are live, two numbers tell you whether the door is still open:
- Bounce rate: keep it under 2%. A rising rate usually means a stale or badly sourced list, so verify addresses before you send.
- Spam rate: keep it under 0.1%, the figure Google asks for. Above that, pause the inbox and fix the targeting before you send again.
Spacing matters too. Emails in a sequence need at least two business days between them, and cold email follow-up covers how many to send and what each one says.
When infrastructure can't help: a perfect setup still lands in spam if the list is wrong. People who never needed your product mark it as spam, and no number of inboxes outruns that. Fix the list before you add domains.
Setting up before the first send
Build the infrastructure before you write the campaign. Buy the domains and set up the inboxes and records. Then let warmup run while you build the list and the sequence. The weeks pass either way, and the campaign launches on inboxes the providers already know.
If you'd rather not build it yourself, outbound email is one of the channels on my startup marketing menu, and the setup includes the sending domains, mailboxes and warmup. Founder-led sales covers where outbound fits while you're still closing deals yourself.
The bouncer remembers faces. Make sure the first ones he sees are good.
Common questions
Can I send cold email from my company's main domain?
Sending cold email from a company's main domain puts that domain's reputation at risk. Spam complaints and bounces count against the domain that sent the mail, so a bad campaign can push ordinary mail like invoices and customer replies toward spam. A separate sending domain keeps that risk away from the domain the business runs on.
Do pre-warmed inboxes skip the warmup period?
Pre-warmed inboxes shorten the warmup period without removing it. A pre-warmed inbox has some sending history, so it can start at a higher volume, but common practice still gives it one to two weeks of ramping before full cold volume. Warmup also keeps running after launch to protect the inbox's reputation.
What's the difference between cold email infrastructure and an email marketing platform?
The difference between cold email infrastructure and an email marketing platform is who the mail goes to. An email marketing platform sends newsletters and campaigns to people who opted in. Cold email infrastructure sends one-to-one messages to people who haven't, from separate inboxes and domains, at a far lower volume per sender.