The formula
The math is simple once you know the constraints:
Safe sending limit per domain: 50–80 emails/day Mailboxes per domain: 2–3 Emails per mailbox per day: 25–35
So: domains needed = (daily email volume) ÷ 65 (midpoint of the safe range).
That's the whole formula. Everything below is the context for why these numbers are what they are and how they scale.
Solo BDR or founder doing outbound
Sending 50–100 emails/day.
2 domains, 4–6 mailboxes. One domain is your primary sender; the other is your backup for rotation. Total cost: ~$25/year in domains + $12–24/month in Google Workspace or Microsoft 365 seats.
This is the minimum viable infrastructure. It works for 3–6 months before you'll want to add a third domain as your first two accumulate sending history and occasional spam complaints.
Team of 5 BDRs
Sending 300–500 emails/day total.
6–10 domains, 15–25 mailboxes. Each BDR gets 2 domains and 4–5 mailboxes assigned to them. You'll want a shared pool of 2–3 extra domains for rotation when someone's domain needs cooldown.
At this scale, manual domain management becomes a real time sink. Tracking which domain is healthy, which needs warmup, which should be rotated — someone on the team spends 2–3 hours/week on this. This is where automation pays for itself.
Team of 15 BDRs
Sending 1,000–2,000 emails/day total.
20–35 domains, 50–80 mailboxes. At this scale, you need a dedicated domain management system — not a spreadsheet. You're buying new domains monthly, retiring burned ones, managing warmup cycles across dozens of mailboxes, and monitoring inbox placement per-domain.
Most companies at this scale either hire a dedicated ops person for email infrastructure or use a platform that manages it natively. The cost of a dedicated ops person ($60–80K/year) vs. the cost of platform-managed infrastructure ($200–500/month) is the economic argument for automation.
Why this is a moat, not a checkbox
Most outbound teams treat domain infrastructure as a one-time setup task. Set up a few domains, configure DNS, start sending. Then they wonder why reply rates drop after month 3.
The teams that win at outbound treat domain infrastructure as an ongoing operational discipline — monitoring, rotating, expanding, and retiring domains continuously. It's the equivalent of maintaining a fleet of delivery trucks vs. buying one truck and hoping it lasts forever.
This is why deliverability compounds as an advantage. A team with 12 months of disciplined domain management has a structural edge over a team that just bought fresh domains. And that edge is nearly impossible to shortcut.
Frequently asked questions
How many emails can I send per domain per day?
50–80 is the safe range for sustained cold email. Above 80, you'll start seeing deliverability degradation within 2–3 weeks, even with warmup running.
How many mailboxes should I create per domain?
2–3 per domain. Each mailbox sends 25–35 emails/day. This distributes sending volume and reduces the chance of any single mailbox triggering spam filters.
Is it worth buying that many domains?
At $10–15/domain/year, the cost is trivial. A team sending 500 emails/day needs ~10 domains — that's $100–150/year in domain costs vs thousands in pipeline if deliverability holds.