How many domains and mailboxes do you need, and what will it cost?
Divide your monthly cold emails by about 30 sends per mailbox per day and 22 sending days to get mailboxes, then by 3 mailboxes per domain to get domains. 20,000 emails a month needs 31 mailboxes on 11 domains. Enter your volume below to compare the full monthly cost on Instantly, Smartlead, Lemlist and WarmHawk.
Mailboxes
31
Domains
11
Infra / month
$228
6,667 new contacts a month. Mailboxes and domains cost the same whichever tool sends, so they are included in every total below.
| Tool & plan | Software | Total / mo | 12 months |
|---|---|---|---|
| Instantly Hypergrowth | $97 | $325 | $3,900 |
| Smartlead Pro | $94 | $322 | $3,864 |
| Lemlist Email | $69 | $297 | $3,564 |
| WarmHawk Tier 1 (dashboard) Flat fee plus your own server. No send, contact or mailbox caps. | $209 | $437 | $5,244 |
| WarmHawk Tier 0 (API only)lowest Free, source-available engine on your server. API only, no dashboard. | $10 | $238 | $2,856 |
List prices, monthly billing, checked 2026-09-29: Instantly · Smartlead · Lemlist · WarmHawk. Annual billing lowers every SaaS price by 17–20%.
Warmup ramp
New mailboxes can’t send 30 a day from day one. With 14 days of warmup, then 5 campaign emails a day growing 20% a day, they reach full volume on day 25.
| Week | Per mailbox / day | All 31 mailboxes |
|---|---|---|
| 1–2 | Warmup only, no campaigns | |
| 3 | 5–15 | 155–465 |
| 4 | 18–30 | 558–930 |
How the math works
- Mailboxes = emails per month ÷ (emails per mailbox per day × sending days), rounded up.
- Domains = mailboxes ÷ mailboxes per domain, rounded up.
- Warmup ramp = at least 14 days of warmup, then 5 campaign emails per mailbox a day, growing 20% a day up to the daily limit. These are WarmHawk’s defaults.
- Software = the cheapest public plan whose monthly email cap and active-contact cap both fit. Contacts = emails ÷ emails per contact.
- Total = software + mailboxes + domains (yearly price ÷ 12). WarmHawk adds your server.
Before you buy domains, run them through the free domain health check. If a campaign starts bouncing, paste the bounce into the bounce decoder. For the reasoning behind the defaults, read how many domains you need for cold email.
Questions
Cold email calculator FAQ
How many emails can one mailbox send a day for cold email?+
Most deliverability guides settle on about 30 cold emails per mailbox per day once the mailbox is warmed, with new mailboxes starting far lower. Google and Microsoft allow much more for normal mail, but cold volume near those limits gets filtered long before it gets blocked. The calculator defaults to 30; change it under Assumptions.
How long before new mailboxes can send at full volume?+
About three and a half weeks. With at least 14 days of warmup, then 5 campaign emails a day growing 20% a day, a mailbox reaches 30 a day on day 25. Buy and connect mailboxes a month before you need their volume; the calculator’s warmup ramp shows the weekly numbers for yours.
How many mailboxes should I put on one domain?+
Two or three. More mailboxes per domain means fewer domains to buy, but a spam complaint or blocklisting hits the domain, not the mailbox, so every mailbox on it suffers together. Keeping it to three per domain limits the damage when one domain goes bad.
Why do all the tools share the same mailbox and domain cost?+
Instantly, Smartlead, Lemlist and WarmHawk all send through mailboxes you own, usually Google Workspace or Microsoft 365 seats on domains you register. Those costs follow you whichever tool you pick, so the calculator adds them to every total rather than making one tool look cheaper by leaving them out.
Is WarmHawk cheaper than Instantly or Smartlead?+
Not at low volume. With 3-step sequences, a $39–$97 SaaS plan beats WarmHawk Tier 1’s $199 flat fee up to about 75,000 emails a month, and Smartlead stays under it until 150,000. Past those points Instantly moves to $358, Lemlist to per-user pricing and Smartlead to $379, while Tier 1 stays $199. The free Tier 0 engine is cheapest at any volume if you can work from its API.
How current are these prices?+
Every SaaS price was read from the vendor's own pricing page on 2026-09-29, monthly billing, and each vendor name above the table links to that page. Annual billing is 17–20% cheaper on all three. Enterprise and custom plans are not shown, because they have no public price.