Free tool
Cold email mailbox and domain calculator
Enter how many leads you want to reach. See how many mailboxes and domains it takes to send that safely.
- Mailboxes
- 5
- Domains
- 2
- Emails per weekday
- 143
3,000 emails a month over 21 weekdays. Values outside the allowed range are adjusted to the nearest limit.
- Good:
Warm up every new mailbox before it sends.
New mailboxes need 2–3 weeks of warmup before cold email, so set them up before you need the volume.
Reachbox spreads each campaign across your mailboxes, keeps every one inside its daily limit and warms them up for free. Start a free trial →
How the numbers are worked out
- Emails per month = new leads per month × emails per lead (your first email plus follow-ups).
- Emails per weekday = that total spread over 21 weekdays, because cold email is best sent Monday to Friday.
- Mailboxes = emails per weekday ÷ how many each mailbox sends, rounded up.
- Domains = mailboxes ÷ mailboxes per domain, rounded up.
Safe defaults
- 30 cold emails per mailbox per day is a safe default. Many teams go up to 50; past that, placement usually drops.
- 2 to 3 mailboxes per domain keeps any one domain from carrying too much risk.
- Send cold email from separate domains (for example getacme.com for acme.com) so your main domain stays clean.
- Every new mailbox needs 2 to 3 weeks of warmup before it sends cold email.
See exactly when each new mailbox can start sending. Warmup schedule calculator →
Nothing you enter leaves this page. The numbers are worked out in your browser.
Questions
How many cold emails can one mailbox send per day?
Keep each mailbox at 30 to 50 cold emails a day once it is warmed up. Google Workspace and Microsoft 365 allow far more in total, but cold email volume well above this tends to land in spam.
How many mailboxes should I put on one domain?
Two or three. If a domain picks up a bad reputation, every mailbox on it is affected, so more domains with fewer mailboxes each spreads the risk.
Should I send cold email from my main domain?
Most teams don’t. They buy a few similar domains for outreach so a reputation problem never touches the domain they use for customers, invoices and hiring.
Do follow-ups count toward the daily limit?
Yes. Every email a mailbox sends counts, which is why the calculator multiplies leads by the number of emails per lead.