How many domains and mailboxes do you need for cold email?

“How many mailboxes do I need?” is the first question every team asks, and most answer it backwards. They pick a number that sounds reasonable, buy the domains, and only later discover their pipeline math never had a chance.

Here is the way we size infrastructure for every client, working backwards from the outcome.

Start from the meetings, not the mailboxes

Say you want 10 qualified conversations a month. Plug in honest conversion numbers and walk it back:

• 10 conversations a month
• At roughly half of positive replies converting to a real meeting, you need about 20 positive replies
• At 30% of replies being positive, that is about 65 to 70 total replies
• At a 3% reply rate, that is roughly 2,200 contacted prospects
• With a 3-step sequence, that is roughly 6,500 sends a month

If your numbers differ, run your own through our ROI calculator. The point is the chain: meetings define replies, replies define volume, volume defines infrastructure.

What one mailbox can actually do

A healthy, warmed mailbox sends 20 to 30 cold emails a day. Use 25 as a planning number and a 22-working-day month, and a mailbox is worth roughly 500 sends a month. Not the 5,000 a sequencer will technically let you queue. 500.

So the 6,500 sends above need about 13 mailboxes. Round up to 15 for slack, because some mailboxes will have bad weeks, and you never want a campaign throttled because the infrastructure is at 100% utilization.

The domain layer

Mailboxes sit on domains, three per domain, so reputation problems stay contained. That makes the domain count automatic:

Sends per month	Mailboxes	Sending domains (3 per)
3,000	6	2
10,000	20	7
25,000	50	17
50,000	100	34

Domains cost about $9 a year at any registrar. The table is why a serious outbound motion budgets domains as a line item, not an afterthought.

Dedicated domains or subdomains?

There are two ways to get those domains:

• Dedicated roots. Every sending domain is its own registration, like getacme.com. Maximum isolation, and what we recommend once outbound is a core channel. This is the $1.00 per mailbox plan.
• Subdomains. Sending subdomains created on a root you already own, one root covering a full block of 50 mailboxes. Nothing new to buy and faster to start, at $1.20 per mailbox. The right choice when you are validating the channel.

The honest guidance: testing cold email for the first time, start on subdomains. Scaling something that already works, go dedicated.

Budget for burn

Even with a perfect setup, some percentage of domains will degrade over a year of sending: a few unlucky complaints, a blacklist sweep, a mailbox provider being moody. Plan on replacing maybe 10 to 15% of domains annually.

This is why we treat replacements as part of the product rather than a surprise invoice. Whoever runs your infrastructure, ask them what a burned domain costs you before it happens, not after.

The shortcut version

• Decide your monthly meeting target
• Walk it back to monthly sends (calculator)
• Divide sends by 500 for mailboxes, round up generously
• Divide mailboxes by 3 for domains
• Add warmup time to your launch plan: 2 to 3 weeks before full volume

Or skip the spreadsheet: capacity on our infrastructure plans is sold in blocks of 50 mailboxes precisely because that is about 25,000 well-behaved sends a month, which covers most serious outbound programs in one or two blocks.