Cold email capacity calculator: how many mailboxes and domains you need

Short answer

Divide the emails you send a month by what one mailbox can safely send: about 30 a day over 22 sending days, or 660 a month. Reaching 1,000 prospects with a 4-step sequence is 4,000 emails, which needs 7 warmed mailboxes across 3 domains at 3 mailboxes per domain. Change the inputs for your volume.

Safe volume per mailbox: About 30 a day, 50 at most
Mailboxes per domain: 2–3, at most 5
Per domain: Under about 500 a day
Spam complaint limit (Gmail, Yahoo): Below 0.3%

New prospects a monthEmails in each sequenceCold emails per mailbox a dayRecommended 30, ceiling 50Sending days a monthMailboxes per domain2–3 recommended

Emails a month4,000

Emails a sending day182

Warmed mailboxes needed7

Sending domains needed3

Outbound Labs planGrowth: 5,000 emails, 5 warmed mailboxes

How the calculator works

Emails a month = prospects × emails per sequence. One mailbox sends emails per day × sending days. Mailboxes = emails a month ÷ that, rounded up; domains = mailboxes ÷ mailboxes per domain, rounded up. Follow-ups spread over several weeks, so a month’s prospects are a fair proxy for a month’s sends once sequences are running.

Why 30 emails per mailbox a day?

No mailbox provider publishes a safe cold email number; the figures come from the sending tools. Instantly recommends 30 campaign emails plus 10 warm-up emails a day per account, Smartlead puts the ceiling at 50, and Smartlead’s own data shows the median mailbox sending about 14 a day. Google Workspace allows 2,000 messages a day and Microsoft 365 10,000 recipients, but those are abuse limits, not cold email targets.

What else does a sending setup need?

Authentication on every domain: SPF, DKIM and DMARC. Check each one with the email domain checker.
Warm-up before volume: two to four weeks per new mailbox; see email warm-up.
Verified lists: keep bounces under 2%; our guide to AI cold email deliverability covers the stop rules.

Outbound Labs includes the warmed mailboxes: 5 on Growth ($149 a month, 5,000 emails) and 25 on Scale ($499 a month, 25,000 emails), capped at 40 emails per inbox a day. Try a free run on your own site or compare cold email software.

Frequently asked questions

How many cold emails can one mailbox send a day?

Most cold email tools recommend about 30 a day per mailbox once it is warmed, and treat 50 as the ceiling. Smartlead’s platform data shows the median mailbox sending about 14 a day. Google Workspace’s hard limit of 2,000 a day is far above what is safe for cold email.

How many mailboxes should I put on one domain?

Two or three, and no more than five. To send more, add domains rather than mailboxes, and keep each domain under about 500 cold emails a day.

Should I send from my main domain?

No. Use separate domains that look like your brand, so complaints or bounces from cold email cannot damage the domain your customers and invoices rely on.

How long does a new mailbox need to warm up?

Plan two to four weeks of gradually rising volume before a new mailbox sends a full cold email load. Our email warm-up page explains the schedule.

Do the Gmail and Yahoo bulk sender rules apply to cold email?

They apply to anyone sending about 5,000 or more messages a day to their users from one domain. Most cold email stays below that, but filters judge everyone on the same signals: SPF, DKIM, DMARC, and spam complaints under 0.3%.

Cold email capacity calculator

How the calculator works

Why 30 emails per mailbox a day?

What else does a sending setup need?

Frequently asked questions

Sources

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