Cold email domains calculator
This cold email domains calculator tells you how many inboxes and sending domains you need for a daily email volume, and what the mailboxes will cost each month. Set your own per-inbox cap and inboxes per domain.
Your inputs are saved only in this page address, so you can bookmark or share them. Nothing is stored anywhere else.
How many domains do you need for cold email?
Divide your daily email target by what one inbox can safely send, then divide the number of inboxes by how many you put on each domain. With example inputs of 500 emails a day, 20 per inbox, and 3 inboxes per domain, you need 25 inboxes across 9 domains. Each step rounds up.
How many cold emails can one inbox send per day?
Community guidance compiled by Deepline puts it at about 15 to 20 a day for a Google Workspace inbox and 10 to 15 for Microsoft 365, and says never to exceed 25 per inbox. These are practitioner benchmarks, not provider rules, and other sources suggest higher numbers for older, well-warmed inboxes.
| Mailbox type | Cold emails per inbox per day |
|---|---|
| Google Workspace | 15 to 20 [1] |
| Microsoft 365 | 10 to 15 [1] |
| Any provider, ceiling | about 25 [1] |
Guidance varies by source. Instantly's help center suggests 30 to 50 emails a day per account [2], and Happierleads suggests 20 to 30 for warm mailboxes and 40 to 50 for mature ones [4]. Starting at the conservative end and raising volume only while bounce and spam rates stay low is the safer path.
How many inboxes should you put on one domain?
Instantly, a cold email platform, recommends 3 to 5 inboxes per sending domain in both its help center and its infrastructure guide. Spreading volume across more domains means a problem on one domain affects a smaller share of your sending. The calculator defaults to 3, the low end of that range, and you can change it.
Are these the providers' sending limits?
No. Providers publish much higher technical limits. Microsoft documents a recipient rate limit of 10,000 recipients per day for an Exchange Online mailbox [5]. Cold email guidance is far lower because sender reputation and spam filtering, not the technical cap, decide whether your messages reach the inbox. Google publishes its own Workspace limits [6].
What does the cost estimate include?
Only mailbox subscriptions: the number of inboxes multiplied by the monthly price per mailbox you enter. It leaves out domain registration, warmup tools, sending software, and contact data. The default price is a placeholder, so enter the current price from your mailbox provider or reseller before you rely on the total.
The formula
inboxes = round up (emails per day / emails per inbox per day)
domains = round up (inboxes / inboxes per domain)
monthly cost = inboxes x price per mailbox
daily capacity = inboxes x emails per inbox per day
Frequently asked questions
Do new inboxes need warmup before cold sending?
Most practitioner guides say yes. Happierleads, for example, recommends no external cold sends during a new mailbox's first 14 days and a gradual increase after that. Leave room in your plan for that ramp-up time.
Should I send cold email from my main company domain?
Many practitioners send from separate secondary domains instead. Instantly's infrastructure guide recommends setting up secondary sending domains for cold outreach.
Why does the calculator round up?
You cannot buy part of an inbox or a domain. Rounding up makes sure your daily capacity covers the target. The daily capacity result shows how much headroom the rounding adds.
Does this calculator store my numbers?
No. The math runs in your browser. Your inputs are written into the page address so you can bookmark or share them. Nothing is sent to a server, and the site uses no cookies or trackers.
Sources
- Deepline GTM Stack: What are the safe sending limits for cold email?. Labels its figures community consensus: Google Workspace 15 to 20 per inbox per day, Microsoft 365 10 to 15, never exceed 25.
- Instantly Help Center: Instantly cold email strategy. Recommends a maximum of 3 to 5 email accounts per domain and 30 to 50 emails a day from each.
- Instantly blog: The cold email infrastructure that gets us 10-20+ positive replies a day. Recommends 3 to 5 inboxes per sending domain and using secondary sending domains.
- Happierleads: Per-mailbox daily send limits for Google and Microsoft. Recommends warmup only in days 1 to 14, 20 to 30 sends a day in weeks 3 to 6, and 40 to 50 for mature mailboxes.
- Microsoft Learn: Exchange Online limits. Recipient rate limit of 10,000 recipients per day.
- Google Workspace Admin Help: Gmail sending limits in Google Workspace. Google's published sending limits for Workspace accounts.
Checked October 25, 2026.