How Many Mailboxes Per Domain for Cold Email

Mailbox providers judge mail at several levels: the individual sender address, the domain, and the sending infrastructure. Domain reputation is the one that quietly links your mailboxes together.

With two mailboxes on a domain, a bad week affects two mailboxes. With ten on a domain, a bad list on one campaign can pull down ten.

Low density also means a single domain is sending a volume that looks like a small team, not a bulk sender.

You can, and some teams do for maximum isolation. The cost is more domains to buy, renew, authenticate, warm, and monitor. Two or three per domain is the middle ground most agencies settle on.

Ten mailboxes on one domain sending 30 a day each is 300 cold emails a day from one young domain. That is a big, sudden footprint. When it burns, it takes all ten mailboxes with it.

1. Daily total sends = new contacts per day multiplied by average sends per contact that are active on a given day. A simple estimate: new contacts per day times 2.5 for a three step sequence once it is running.
2. Mailboxes needed = daily total sends divided by sends per mailbox per day.
3. Domains needed = mailboxes needed divided by mailboxes per domain.
4. Add 10 to 20 percent spare for resting or replacing mailboxes.

An agency wants to reach 400 new contacts per day for a client with a three step sequence.

Daily total sends: 400 times 2.5 is about 1,000.
Sends per mailbox: 30.
Mailboxes: 1,000 divided by 30 is about 34.
Mailboxes per domain: 3.
Domains: 34 divided by 3 is about 12.
With spare capacity: roughly 38 to 40 mailboxes on 13 or 14 domains.

If that feels like a lot, compare it with the cost of a campaign going quiet because five overloaded domains burned in a month.

Google Workspace allows far more than any cold email team should send. The published per user limit on paid accounts is in the thousands per day. That limit is about abuse prevention, not placement. For cold email, the operating range most teams use is:

Weeks 1 to 3: warmup only, no campaigns.
Weeks 3 to 4: roughly 10 to 20 cold sends per mailbox per day.
Steady state: roughly 20 to 40 per day, depending on list quality and reply rates.

Increase gradually. If replies fall or bounces rise, lower the cap before adding anything else.

Go lower density or lower volume if: Domains are brand new. Lists are broad or less verified. The offer generates more complaints. You are sending for a client whose brand risk is high.

You can go a little higher if: Domains have months of clean history. Lists are tightly targeted and verified. Reply rates are healthy and complaints near zero.

Even then, add capacity with more domains rather than more mailboxes per domain.

Instantly, Smartlead, and similar tools rotate across mailboxes. Rotation only spreads risk if the mailboxes are spread across domains. If 30 mailboxes sit on three domains, rotation is spreading volume across three reputations, not thirty. When you set up a campaign, check how many distinct domains it is actually using.

Keep a simple table per client or brand: domain, mailbox count, daily cap per mailbox, warmup start date, status. Review weekly: Any domain above your target density? Any domain with reply rate clearly below its siblings? Enough spare mailboxes warmed and ready?

ColdMail provisions official Google Workspace mailboxes on dedicated domains, with SPF, DKIM, DMARC, and MX published automatically, which makes a low density setup practical: adding a domain is not another afternoon of DNS. Each domain has its own official admin panel with 2FA, and domain and mailbox health views help you spot one domain drifting. AI warmup is available as an add on. Book a Strategy Call at coldmail.app to map a domain and inbox plan for your target volume.