How many mailboxes and domains you need for cold email

To work out how many cold email mailboxes you need, start with the number of emails you want to send each day, follow-ups included. Divide that by a safe daily volume per mailbox, which in common practice sits somewhere between 10 and 50. Add some spare capacity. Then divide by the number of mailboxes you put on each domain, usually 2 or 3, and you have your number of cold email domains.

A quick example: 500 emails a day at 25 per mailbox means 20 active mailboxes. With 25% spare capacity that becomes 25 mailboxes, which fits on 9 domains at 3 mailboxes each. None of those domains should be your main company domain.

Step 1: know your real daily volume

The number that matters is total sends per day, not new prospects per day. Every follow-up is an email that some mailbox has to send.

If each prospect receives a short sequence of 3 emails, then once the sequence runs at a steady pace you send roughly 3 emails for every new prospect you add each day. Adding 150 new prospects a day means about 450 sends a day, a little less as people reply or ask you to stop. If you are still deciding how many follow-ups to send, see how to write a short B2B cold email sequence.

Also check that the volume fits your market. If your target list holds 3,000 companies and you contact 200 new ones a day, you run out in 15 working days. Settle who you are targeting before sizing any infrastructure.

Step 2: pick a safe volume per mailbox

A mailbox here means one sending address, such as one user on Google Workspace or Microsoft 365.

The provider limit is a ceiling, not a target

Google and Microsoft both publish technical sending limits. A paid Google Workspace user can send up to 2,000 messages a day, to at most 3,000 external recipients and 2,000 unique external recipients. An account converted from a trial gets these higher limits only once the domain has paid at least USD 100 in total, and Google notes the increase can take up to 75 days after that. Trial accounts are capped at 500 messages a day. A user who goes over can't send new messages for up to 24 hours (Gmail sending limits in Google Workspace).

Microsoft 365 allows 10,000 recipients per mailbox per day and 30 messages per minute. On top of that, a tenant-wide cap on external recipients depends on how many licenses you hold, so adding mailboxes does not raise your capacity without limit (Exchange Online limits).

These numbers describe what the system will accept. They say nothing about what is safe for unsolicited email to strangers. Microsoft states that its recipient limits exist "to discourage the delivery of unsolicited bulk messages." A mailbox that sends hundreds of cold emails a day will usually run into spam filtering long before it reaches any technical cap.

A working range for cold email

There is no official number for cold email, and the figures in circulation vary a lot. A common practice range is 10 to 50 cold emails per mailbox per day, follow-ups included. Newer mailboxes and cautious senders sit near the bottom of that range.

Google's sender guidelines ask senders to send at a consistent rate and to avoid sending in bursts (Gmail email sender guidelines). In practice, that means spreading each mailbox's sends evenly through the working day rather than sending them all at once.

The figure you choose drives everything else. For 500 emails a day:

• At 10 per mailbox, you need 50 mailboxes.
• At 25 per mailbox, you need 20 mailboxes.
• At 50 per mailbox, you need 10 mailboxes.

If your mailbox provider or sending software sets its own daily cap, use the lower of the two numbers.

Step 3: decide how many mailboxes per domain

Mailbox providers judge incoming mail partly on the reputation of the domain it comes from. Mailboxes on the same domain share much of that reputation. If one of them draws spam complaints or hits a batch of invalid addresses, the whole domain can start landing in spam, and every mailbox on it is affected. People call this shared risk the blast radius.

So mailboxes per domain is a trade-off:

• Fewer mailboxes per domain (1 or 2) keep the blast radius small. Losing one domain costs you little capacity. In exchange, you have more domains to buy and set up, and more mailboxes to warm up, which means gradually building a new mailbox's sending history before it carries cold email.
• More mailboxes per domain (4 or 5) mean fewer domains and less setup work, but one incident takes out a larger share of your sending at once.

Most guidance sits at 2 to 3 mailboxes per domain. Ratios of 4 or 5 do appear, but treat them as the risky upper end rather than a neutral choice. Nothing official sets the number, and some mailbox providers recommend or enforce their own maximum, so check theirs first.

One helpful way to compare options is the daily volume per domain: mailboxes per domain multiplied by emails per mailbox. Three mailboxes at 25 a day send 75 cold emails a day from one domain. Five mailboxes at 10 a day send 50. A lower volume per domain does not remove the shared risk, which grows with the number of mailboxes on the domain.

Step 4: count your domains

Domains equal mailboxes divided by mailboxes per domain, rounded up.

The sender guidelines linked above set no domain count. What matters is how each domain is set up:

• Names that look like your brand. A credible variant reads as yours: your brand with a short prefix such as "try" or "get", or the same name on another extension, such as example.net for a company at example.com (illustrative). Random strings of letters, hyphens and numbers look like throwaway domains.
• Something behind each domain. A domain that exists only to send mail can look suspicious. Pointing it to your main site avoids an empty domain.
• Familiar extensions. A .com or a familiar country extension is a safer choice than a cheap novelty one.
• A clean technical setup on every domain. The records involved are covered in cold email deliverability basics.

Never send cold email from your main domain

Your main domain carries the email that keeps the company running: replies to customers, invoices, password resets, product notifications, messages to investors and candidates. If cold outreach damages its reputation, that mail can start landing in spam too, and a damaged domain reputation is slow to repair.

Secondary domains are meant to be replaceable. If one gets burned, you retire it and switch to a warmed spare. You cannot do that with the domain on your website and your contracts.

A subdomain such as outreach.example.com (illustrative) is not fully separate either. Google counts messages sent from subdomains toward the primary domain when it decides whether you are a bulk sender, meaning a sender of close to 5,000 messages or more to personal Gmail accounts within 24 hours, who must then meet stricter rules. Once applied, that classification is permanent (Google email sender guidelines FAQ).

Separate domains give you more separation than a subdomain. They do not guarantee that your main domain's reputation is protected, especially when they carry your brand name and point to your site.

Keep spare capacity

At any given time, some of your mailboxes will not be sending at full volume:

• new mailboxes that are still warming up
• mailboxes paused after a warning sign, such as a rise in bounces
• mailboxes on a domain you are retiring and replacing

If you size your setup to exactly your target, each of these events puts you below it. A common practice is to add 20 to 25% on top of the mailboxes your daily volume requires.

A new mailbox needs time before it can carry cold email (see how long to warm up a new mailbox). That means spares have to be bought and warming before you need them, not on the day a domain starts to struggle.

A worked example

Say you want to add about 165 new prospects a day to a 3-email sequence. The numbers below are illustrative. Round up to whole mailboxes first, then add spare capacity.

1. Daily volume. 165 × 3 = 495, so roughly 500 sends a day.
2. Active mailboxes. At 25 emails per mailbox per day, 500 ÷ 25 = 20 mailboxes.
3. Spare capacity. Adding 25%, 20 × 1.25 = 25 mailboxes.
4. Domains. It depends on your ratio:
   • at 2 per domain, 25 ÷ 2 = 12.5, so 13 domains
   • at 3 per domain, 25 ÷ 3 = 8.3, so 9 domains
   • at 5 per domain, the risky upper end, 25 ÷ 5 = 5 domains

Now lower the per-mailbox volume to 15 a day. 500 ÷ 15 = 33.3, which rounds up to 34 active mailboxes. Adding 25%, 34 × 1.25 = 42.5, so 43 mailboxes. At 3 per domain, 43 ÷ 3 = 14.3, so 15 domains instead of 9. The per-mailbox figure sets how many mailboxes you need, and the per-domain ratio only decides how they are grouped into domains.

You can also start below your target and add domains once the first sends show the volume is worth it.

Key takeaways

• Mailboxes needed equal daily sends, follow-ups included, divided by a safe volume per mailbox, rounded up, plus 20 to 25% spare.
• Domains needed equal mailboxes divided by mailboxes per domain, rounded up.
• Plan on 10 to 50 cold emails per mailbox per day, a practice range, not an official rule. Provider sending limits are ceilings, not targets.
• Most guidance puts 2 to 3 mailboxes on each domain. Fewer means a smaller blast radius, and 4 or 5 is the risky upper end.
• Never send cold email from your main domain. A subdomain does not keep it separate, and separate domains reduce the risk without removing it.
• Warm your spare mailboxes before you need them.