What you need to calculate mailbox capacity
The calculator does not need an estimated open rate or a promise of leads. It needs the first-wave volume, number of active sending days, touches in the sequence, daily capacity per mailbox, mailboxes per domain and reserve capacity. Clean the list before calculating: an invalid address creates a bounce, not useful capacity. Start with email list verification.
- Volume: the number of unique email addresses that will receive the first email.
- Touches: every email planned for one contact in the sequence.
- Sending days: actual sending days, not calendar days in the month.
- Daily mailbox capacity: the limit set in your own operating policy.
- Reserve: mailboxes and domains kept ready to replace working infrastructure when needed.
How the calculator turns campaign volume into domains
First, it calculates the daily flow by dividing the total email volume by the sending days. Total volume includes first emails and follow-ups: if a contact receives several touches, the whole sequence belongs in the calculation. The daily flow is then divided by the capacity of one mailbox to establish the minimum number of working mailboxes. Finally, those mailboxes are divided by the number assigned to each domain, and reserve capacity is added.
| Input | What to include | Why it matters |
|---|---|---|
| Sequence volume | Addresses and every planned touch | Shows the real number of sends |
| Sending pace | Active sending days and mailbox capacity | Determines mailbox count |
| Domain allocation | Mailboxes assigned to one domain | Determines domain count |
| Reserve | Additional ready infrastructure | Keeps the campaign moving during a replacement |
Why calculating from the raw list often gives the wrong answer
A common mistake is treating the number of rows in a CRM export as the sending volume. The file may contain duplicates, generic inboxes, former employees and companies you have already contacted. Another mistake is counting only the first email: a reply may arrive after a later touch, while your infrastructure carries the load of every send. The order should be deduplication, address verification, segmentation, then calculation. Read more about the touch logic in Outreach Sequence: How It Works.
A commercial director shares a company list and asks to contact everyone. The calculation uses the row count, even though one company may have several contacts and some rows are repeat contacts. Once the list is cleaned, both the sending schedule and mailbox requirement change. Calculate from verified addresses and the planned touch sequence, not from the size of the original file.
How to use the result
- Check that the calculated working mailboxes can carry the first wave and follow-ups at the same time.
- Keep working infrastructure separate from reserve capacity; reserve is not part of the planned daily flow.
- Check the domain’s DNS configuration before launch: SPF, DKIM, DMARC and mail routing must be in place.
- Match the pace to the readiness of your list and reply handling. More mailboxes do not solve a campaign if nobody can qualify incoming conversations.
If the calculator returns a high domain count, do not compress the volume into a few mailboxes. Review the campaign timeline, first-wave size or sequence structure instead. Before sending, use the SPF, DKIM and DMARC setup guide and review domain and mailbox warm-up.
When this calculation is not for you
This calculator cannot tell you whether your offer will interest the market or replace ICP validation. It is not designed for a one-off send with no follow-ups, a list with no clear provenance, or a campaign where Telegram or LinkedIn is the main channel. If you are testing a hypothesis with a small list of target companies, account selection and a personal approach matter more than infrastructure scale; that is an account-based marketing task.