What infrastructure does your target need?
Enter the leads or emails you want per month and get the mailboxes and domains to buy — or go the other way and check what your current fleet can safely send. No signup.
Your inputs
Fill in your real setup
Total mailboxes actively sending cold email.
20–30 is the safe range for warmed mailboxes.
Used to check your mailboxes-per-domain risk.
Each new prospect consumes one send per step.
Your safe capacity
What your setup can safely send
Warmed mailboxes on domains older than 30 days. New setup? Warm up first.
This tool is for guidance only. Your results may vary.
How the math works
Mailboxes needed = daily sends ÷ 25. A warmed mailbox safely carries 20–30 cold sends a day — push past that and placement quietly degrades long before anything looks broken. Full breakdown of daily limits here.
Domains needed = mailboxes ÷ 3. Mailboxes on one domain share its reputation, so one burned inbox drags the rest down with it. Three or fewer per domain keeps that blast radius small.
A monthly figure divides by 21 sending weekdays, not 30 days — cold email does not run at the weekend, and a 30-day month overstates capacity by nearly a third. Lead volume divides monthly sends by your sequence length: a 5-step sequence means each new prospect consumes five sends. Replies assume the typical 2–5% total reply rate; your list quality decides which end you get.
The number this calculator can't give you: how much of that fleet is actually healthy right now. A mailbox that silently disconnected or a domain drifting toward a blacklist takes real capacity to zero while the plan says otherwise — that gap is what Coldops watches across every client workspace.
Cold email infrastructure questions
How many mailboxes do I need for cold email?+
Divide your daily send target by 20–30, the safe range for one warmed mailbox. To send 500 cold emails a day you need roughly 17–25 mailboxes. To reach 1,000 new leads a month on a 5-email sequence you need about 5,000 sends a month, which is 240 a day across 8–12 mailboxes.
How many mailboxes per domain is safe?+
Three or fewer. Mailboxes on one domain share that domain's reputation, so a single burned inbox drags the others down with it. Splitting 12 mailboxes across 4 domains contains the damage; putting all 12 on one domain does not.
How many domains do I need for cold email?+
Divide your mailbox count by 3. A 24-mailbox fleet needs 8 sending domains. Buy them as separate registrations rather than one bulk batch on a single registrar and nameserver — blocklists key on that pattern.
How many cold emails can one mailbox send per day?+
20 to 30 once it is warmed. Google and Microsoft moved from spam-foldering to outright rejection for non-compliant senders, which roughly halved the volume a single inbox could safely carry. Past 30 a day, placement degrades quietly before anything looks broken.
How long before new mailboxes can send cold email?+
About 21 days of warmup. A fresh domain and mailbox have no sending history, so volume has to ramp gradually. Buy infrastructure roughly three weeks before you need it sending — the fleet a calculator sizes is what you need live, not what you can send from tomorrow.
How many leads do I need per month for cold email?+
Divide your monthly send volume by your sequence length. A 5-email sequence at 5,000 sends a month consumes 1,000 new leads. Longer sequences need fewer new leads for the same volume, which is the cheapest lever when list supply is the constraint.