How the mailbox math works
Start with the number of new emails you need to send each day, not per month. Divide by the daily limit you are willing to put on one mailbox, then round up. That is the raw mailbox count. Add a buffer on top, because in practice some mailboxes are always paused, cooling off after a bounce spike, or busy handling replies rather than sending.
mailboxes = ceil( ceil(target_per_day / sends_per_mailbox) x (1 + buffer) )
domains = ceil( mailboxes / mailboxes_per_domain )
Example: 1,000 a day at 30 per mailbox with a 20% buffer
= ceil(34 x 1.2) = 41 mailboxes = 14 domains at 3 per domainMonthly volume is the daily target multiplied by sending days. Five days a week works out to about 21.7 sending days a month, so 1,000 a day is roughly 21,700 emails a month, not 30,000. Plan your list size and your sending tool's limits around that number.
Why 20 to 30 sends per mailbox is the safe range
Google Workspace allows far more than 30 outbound emails a day, but the limit that matters for cold email is reputation, not the hard cap. New mailboxes sending unsolicited mail to strangers get judged on complaint rate, bounce rate and engagement. Keeping each inbox at a modest, human-looking volume gives every mailbox room to absorb a few bad days without a block.
- Count warm-up traffic and follow-ups towards the daily limit, not just first touches.
- Spread sends across the working day with randomised gaps rather than firing them all at 9 am.
- Keep two or three mailboxes per domain so a single problem inbox cannot take the domain down with it.
- Rotate new domains in gradually; do not launch fifteen fresh domains on the same morning.
If your target volume needs more mailboxes than you expected, that is the correct answer. The alternative, pushing 60 or 80 a day through fewer inboxes, usually costs more in burned domains than it saves in subscriptions.
What the infrastructure actually costs
The two recurring lines are mailboxes and domains. Google Workspace mailboxes bought through a reseller typically land in the low single digits per mailbox per month, and a .com renews for around the price of a sandwich each year. Domains look cheap on paper, but once you need dozens the registration, DNS setup and renewals add up in time as much as money.
The calculator lets you edit both unit prices so you can compare your current provider against a reseller or an all-in platform. It excludes your sending tool, data and labour, which the ROI calculator covers. InboxOne bundles mailboxes with automatic SPF, DKIM, DMARC and MX so the domain count is the only DNS you never have to touch.
Warm-up changes the timeline, not the count
New mailboxes cannot start at their full daily limit. During warm-up each inbox ramps from a handful of sends to its target over two to four weeks, which is why the chart shows available volume climbing towards the target line. If you need full volume on a fixed date, work backwards and start provisioning early, or buy pre-warmed mailboxes that skip most of the ramp.
Because volume is lower during warm-up, the same mailbox count delivers noticeably fewer emails in month one than in month two. Set expectations with clients and reps accordingly, and treat the first weeks as reputation building rather than pipeline building.
