Warm every inbox for three to four weeks before it carries live volume.
Costs use $2.50 to $6 per inbox a month and $10 to $15 per domain a year (Maildeck, 2026). Spares are 20% of live domains.
How the calculator works
The number of inboxes you need comes from two inputs, not from a vendor's package. Divide your daily send target by how many emails one inbox can safely send a day. Then divide the inbox count by how many inboxes you are willing to put behind one domain. Add spare domains on top, warmed and idle, so one flagged domain never takes a live campaign offline.
- Sends per inbox: around 20 a day while domains are new, 25 once warmed, up to 35 or 40 on aged domains or a dedicated IP.
- Inboxes per domain: two or three. Fewer per domain means more domains but caps how much volume one bad domain can take down.
- Spares: 15 to 20 percent of your domains, kept warm for rotation.
The full reasoning, with a worked table and the costs, is in how many domains and inboxes you need, by volume. The parts underneath it, from authentication to monitoring, are in outbound infrastructure architecture. Once the domains exist, check each one with the SPF, DKIM and DMARC checker.
FROM THE YARD
The most common sizing mistake is planning for the aged-domain number from day one. New domains hit a wall at around 15 to 20 a day, and the campaign stalls right when it needs volume. Size for where your domains are now, not where they will be.
Questions about sizing cold email infrastructure
How many emails can one inbox send per day?
Around 20 to 30 cold emails a day once the inbox is warmed. New inboxes should stay lower while they warm up. Aged domains with a strong history can carry a little more.
Why not put more inboxes on one domain?
Because every inbox on a domain shares its reputation. If the domain gets flagged, all of its inboxes stop working at once. Two or three per domain keeps the damage small.
Can I use subdomains instead of new domains?
Subdomains share reputation with the main domain, so they do not add the same headroom. Separate domains are the safer way to spread volume.