Thirty mailboxes across ten domains, if your replies come in at 2%. At 1%, the same setup books about ten calls, and you need the 50 or 100 mailbox tier to reach twenty. That is the answer. The rest of this article shows the arithmetic behind it so you can check it against your own numbers.
The math below works backwards from a booked-call target to the mailbox count, the domain count and the monthly cost, so you can price the setup before you commit to it.
Start from what one mailbox can do
A warmed mailbox that sends more than about 20 emails a day starts getting flagged, so 20 is the ceiling we plan around. Over 22 sending days that is 440 emails a month per mailbox. Each lead gets two emails, the first message and one follow-up two days later, so one mailbox reaches 220 new people a month.
Mailboxes live three to a domain. That keeps any single domain's volume low enough that one bad week of bounces doesn't take the whole sending pool down with it.
Then run the funnel
We plan cold email on five conversion rates. The first one is the only one that moves much from campaign to campaign.
- Reply rate. 1% of leads on a conservative campaign, 2% on a strong one. This counts unique people who reply, not emails.
- Positive replies. 30% of all replies. The rest are out-of-office, unsubscribe, or no.
- Booked. 50% of positive replies turn into a call on the calendar.
- Show rate. 70% of booked calls happen.
- Close rate. 30% of the calls that happen turn into a deal.
Show and close belong to your sales process, not ours. The first three are what an outbound system controls.
Put the two together
Thirty mailboxes reach 6,600 people a month. At a 1% reply rate that is 66 replies, 20 positive, and 10 booked calls. At 2% it is 132 replies, 40 positive, and 20 booked calls.
So the answer to "how many mailboxes for 20 calls" is a range, and it depends on which reply rate you earn:
- 10 mailboxes on 4 domains reach 2,000 people and book 3 to 6 calls.
- 30 mailboxes on 10 domains reach 6,600 people and book 10 to 20 calls.
- 50 mailboxes on 17 domains reach 11,000 people and book 17 to 33 calls.
- 100 mailboxes on 34 domains reach 22,000 people and book 33 to 66 calls.
We provision in those tiers rather than back-solving to an odd number like 37 mailboxes. Domains come in blocks of three mailboxes, and sending platforms price by volume tier, so a clean tier costs the same as the odd number just below it.
What it costs
The infrastructure for 30 mailboxes runs about $224 a month plus $130 a year in domains. That breaks down as $99 for the mailboxes, $99 for the sending platform, and about $26 to verify 6,600 addresses before anyone sends to them. Domains cost $13 each per year.
Stepping up to 50 mailboxes is about $307 a month and $221 a year in domains. At 100 mailboxes it is about $486 a month and $442 in domains.
None of those figures include the list. Sourcing 6,600 to 22,000 qualified people a month is its own job, and it is usually the harder one.
Why the reply rate is the number to argue about
Everything above swings on that 1% versus 2%. Doubling the reply rate does the same work as doubling the mailboxes, at zero extra infrastructure cost. Reply rate comes from the list and the offer, whether the people you emailed can buy what you sell, and whether the first line gives them a reason to answer. We treat the sending setup as a fixed cost and spend the effort on the list and the copy.
Run the same steps against your own target and the number falls out. If you want to see what the running system looks like from the inside, the cold email console is in the playground. If you'd rather have us stand it up and run it, book a call.