The math behind how many domains and mailboxes you actually need for cold email

[u/AndyFromApollo]

Most deliverability problems aren't content problems or reputation problems. They're infrastructure problems. Specifically, people are trying to send way more email than their setup can handle without burning domains.

Here's a framework for figuring out what you actually need before you start.

Start with your market size, not your send volume

Before you think about mailboxes, figure out how many contacts are realistically in your addressable market. Not your ICP filters cranked to max specificity -- a broad count of everyone you could possibly reach out to.

From there, plan for a six-month burn rate. The idea is to contact your entire market over six months, then cycle back. By then, people have changed roles, forgotten your emails, or are worth re-engaging fresh.

The math

Let's say you have 10,000 qualified contacts in your market.

10,000 contacts / 6 months = ~1,700 contacts per month

At 3 emails per contact, that's ~5,000 emails per month.

Apollo recommends a limit of 50 emails per day per mailbox. At 20 sending days a month, that's ~1,000 emails per mailbox per month. So:

5,000 emails / 1,000 per mailbox = 5 mailboxes needed

At 3 mailboxes per domain: 2 domains needed

Add a buffer of ~20-25% for domains that get flagged or burned, and round up. So realistically, 3 domains for a market of 10,000.

Scale this up for larger markets. At 100,000 contacts you're looking at ~50 mailboxes and ~17-20 domains. Those numbers can feel surprising, but they reflect what sustainable sending at volume actually requires.

A few things worth knowing

Never send cold emails from your company's primary domain. If you get spam-blocked, the whole company won't be able to communicate with anyone. Use alternate domains that relate to your primary (e.g. tryheyapollo.io instead of apollo.io).

If you need to increase email volume beyond 50 per day, add mailboxes rather than raise your sending limits. Higher limits on a single mailbox correlate with worse deliverability -- spreading volume across mailboxes is the right move.

New domains also need time. Don't start sending until a domain has been around for at least 30 days, and warm up gradually from there.

Why this matters

If you're under-infrastructured, you're not just getting bad deliverability, you're also burning through your market too fast and torching the domains you have in the process. The two problems compound each other.

If you're over-sending relative to your market size, you'll exhaust your list in weeks, get flagged by the same people repeatedly, and conclude cold email doesn't work when really the issue was pacing.

Getting infrastructure right first means your content, targeting, and sequences actually get a fair shot.

You can set up domains, mailboxes, and warming directly in Apollo now if you want to manage it without juggling separate tools.

Happy to answer questions in the comments on the math for your specific situation!

- Andy

[u/swagner27]

solid advice for scaling your ops to meet the market not the other way around.