A simple formula for how many mailboxes and domains your cold email volume needs, with a table from 500 to 10,000 emails a day and what each setup costs.
Too few mailboxes and each one sends too much and burns. Too many and you pay for capacity you never use. The right number comes from one short formula.
The formula
Mailboxes = daily sends ÷ sends per mailbox, plus 20%. Domains = mailboxes ÷ 3.
Keep each mailbox around 30 cold emails a day once warm. That leaves room for warmup traffic and replies without looking like a bulk sender. Three mailboxes per domain spreads risk, so one bad day never takes down your whole estate. InfraOptimal caps each domain at five and recommends three.
From contacts to daily sends
Most teams think in contacts per month. Convert with: contacts × steps in the sequence ÷ 22 working days. 5,000 contacts on a four-step sequence is 20,000 emails, or about 910 a day.
The table
| Emails a day | Mailboxes / domains |
| 500 | 17 / 6 |
| 1,000 | 34 / 12 |
| 2,500 | 84 / 28 |
| 5,000 | 167 / 56 |
| 10,000 | 334 / 112 |
Counts include roughly 10% headroom over the straight division. Add a further buffer of warming mailboxes if you rotate often.
What each setup costs on InfraOptimal
| Mailboxes, monthly billing | Per month |
| 17 at $1.90 | $32 |
| 34 at $1.90 | $65 |
| 84 at $1.80 | $151 |
| 167 at $1.80 | $301 |
| 334 at $1.70 | $568 |
Every mailbox sends from a dedicated IP included in the price. Annual billing takes the rate down to $1.00 at 500 and up. The minimum is 10 slots.
When the send cap changes the maths
Some providers cap each mailbox at 10 or 15 cold emails a day. At 15 a day, 1,000 emails needs 67 mailboxes and 23 domains instead of 34 and 12. Always compare setups at the same daily volume, not the same mailbox count.
Ramp before you scale
New mailboxes need three to four weeks of warmup before full volume. Start at 5 to 10 sends a day, add a few each week, and only count a mailbox toward your capacity once it is at 30.
Plug your own volume into the cold email setup calculator or read what cold email infrastructure costs.
FAQ
How many emails can one mailbox send a day?
Around 30 cold emails a day once warm is the safe working number. New mailboxes start at 5 to 10 a day.
How many mailboxes per domain?
Three is the common recommendation. InfraOptimal allows five and recommends three.
Do I need more mailboxes on a shared pool?
Only if the provider caps daily sends. A 15-a-day cap doubles the mailboxes you need for the same volume.
One platform instead of a stack of tools
Campaigns, unlimited warmup, verified leads and a unified inbox, plus InfraOptimal mailboxes on a dedicated IP from $1.00. Everything you need to land in the inbox and book more meetings.
Put this into practice on your own IP
InfraOptimal is our sending platform for cold outreach: dedicated IPs, DNS written for you, warmup from day one. From $1.00 a mailbox, about a quarter of the Google and Microsoft price.
| 100 mailboxes a month | Cost |
| InfraOptimal, dedicated IP | $100 / mo |
| Google Workspace, shared IP | $400 / mo |
| Microsoft 365, shared IP | $400 / mo |
Dedicated IPs only · 10 mailbox minimum · SMTP and IMAP included