# How many sending domains and mailboxes does outbound need?

> Work backwards from how many emails you want to send each day. This post shows the arithmetic, how long warming takes and what to set up before the first send.

- Canonical page: https://warmup.ai/blog/how-many-sending-domains-and-mailboxes-does-outbound-need
- By WarmUp. Published 2026-10-27.
- Category: GTM engineering (https://warmup.ai/blog/category/gtm-engineering)
- Part of: GTM engineering (https://warmup.ai/gtm-engineering)
- Tags: deliverability, sending domains, mailboxes, warm-up

## Key takeaways

- The number of mailboxes comes from the emails you want to send each day divided by the cap you set for one mailbox. The number of domains follows from how many mailboxes you put on each.
- Send outbound from separate domains, never from your main one, so a deliverability problem cannot reach the mail your team sends to customers.
- New domains need two to three weeks of warm-up before they carry real volume, so order them on day one, before the copy is finished.
- Set up authentication first, watch inbox health every day, and keep the spam rate far below the line Google publishes.

An outbound program needs enough mailboxes that none of them sends more than its daily cap, and enough sending domains that each domain carries only a few mailboxes. You work it out backwards from how many emails you want to send each day, and you order the domains first, because they need two to three weeks of warm-up.

The rest of this post shows the arithmetic and the setup.

## Why do you send outbound from separate domains?

You send outbound from separate domains so that a deliverability problem stays away from your main one. Mailbox providers judge a sending domain by how recipients treat its mail. If cold outreach earns complaints on your company domain, the mail your team sends to customers can suffer for the same reason.

The fix is to buy sending domains that are close to your company name, set them up properly, and use them only for outbound. Replies still reach your team, and the main domain never sends a cold email.

This is a standard part of how we build outbound at WarmUp: separate sending domains, so the main domain is never at risk. It is also why the infrastructure is set up on day one and not left until the copy is ready.

## How do you work out how many mailboxes you need?

Divide the emails you want to send each day by the number one mailbox is allowed to send. The result, rounded up, is the mailbox count. The cap is your decision and should be conservative, because a mailbox that sends too much too soon puts the reputation of a new domain at risk.

Here is a hypothetical example, with assumptions you should replace with your own.

1. A team wants to start 250 new conversations a week, each with a three-email sequence. That is 750 emails a week.
2. Spread over five working days, that is 150 emails a day.
3. Assume each mailbox sends no more than 30 emails a day. The figure 30 is an assumption for this example, not a rule. Use the cap your sending tool and your own results support.
4. 150 divided by 30 is five mailboxes.

If the team wants to double the pace, it can double the mailboxes or it can wait twice as long. The list length does not change the mailbox count. It changes how many weeks the program runs.

## How many domains do you need?

Divide the mailboxes by the number you are willing to put on one domain. A small number per domain limits how much one problem can affect, because the damage from a bad week stays on that domain. Round up, and keep a spare so you can rest a domain without stopping sending.

Continuing the example, assume two mailboxes per domain. Five mailboxes need three domains, one of which would carry a single mailbox. The figure two is again an assumption for the arithmetic.

More domains is not always better. Each one is another thing to register, authenticate, warm and watch every day. A team that cannot monitor twenty domains is safer with five that it can.

## How long does warming take?

Plan for two to three weeks before a new domain carries real volume. Warming means sending a small, steady amount of mail that gets read and answered, so mailbox providers see a new sender behaving like a real one, then raising the volume in steps.

That timing is why we order sending domains on day one of an engagement. It is the slowest step in the build and the only one that cannot be sped up by working harder. Copy, lists and signals can all be finished while the domains warm in the background.

During warm-up, keep volume low and steady, and do not add cold prospects until the domain is ready. Treat warm-up as a state the domain has to keep, and not as a task you tick off once.

## What has to be set up before the first send?

Authentication has to be in place first. Google's sender guidelines ask for SPF and DKIM, plus DMARC, on the sending domain, and they say marketing messages must support one-click unsubscribe. Those requirements are written for bulk senders, but they describe what a mailbox provider expects from a legitimate sender at any size.

In plain terms, SPF lists which servers may send for the domain, DKIM signs each message so it can be shown to be unaltered, and DMARC tells receivers what to do when a message fails. Set all three before warm-up starts, and test with a message to yourself.

Read the source for yourself. The [Google email sender guidelines](https://support.google.com/a/answer/81126) are short and specific, and they set out plainly what one major mailbox provider checks. Other providers publish their own.

## How do you know the infrastructure is healthy?

Watch it every day, and set limits that trigger a pause. Google's guidelines say to keep the spam rate reported in Postmaster Tools below 0.10% and to avoid ever reaching 0.30%. Treat the first figure as your target, and pause a mailbox long before the second.

A daily check should cover four things: bounces, replies that complain, whether test messages reach the inbox, and the volume each mailbox actually sent. The aim is to hear about a problem from your dashboard and not from a sales rep who has noticed that replies stopped.

We watch inbox health daily for the systems we run, so a problem is fixed before it reaches your numbers. That is part of the [GTM engineering](https://warmup.ai/gtm-engineering) work: the infrastructure, the lists and the plays are built as one system, so a deliverability problem shows up early and has a cause you can find.

## What should you do first?

Order the domains today. They are the slow part, and nothing else in the program has to wait for them. While they warm, write down your daily send target, your per-mailbox cap and your per-domain mailbox count, so the arithmetic above is done on your numbers and not on ours.

Then set up authentication, test it, and start warm-up at low volume. When the domains are ready, launch with a short list of accounts that have a reason to hear from you, and watch the first week of results before raising the pace.

## Questions, answered

### Can I send cold email from my main company domain?

We advise against it. If outbound damages the reputation of that domain, the mail your team sends to customers and prospects suffers too. A separate sending domain keeps the risk away from your main one. It should still look like you, so buyers who look it up recognize the company.

### Does a bigger list mean I need more mailboxes?

Only if you try to send it faster. A longer list can be sent over more weeks from the same mailboxes. The count is set by how many emails you send each day, so decide the pace first and let the list length decide how long the program runs.

### What should I do when a mailbox starts landing in spam?

Pause it, find out why and fix that before sending again. Check authentication, the last few days of volume, the bounce rate and recent replies for complaints. Pausing one mailbox early costs a few days. Pushing through can cost the whole domain.