How many domains and mailboxes do you need for cold email?
A simple way to size cold email infrastructure from your daily sending target, with the ratios SageLift uses for planning.
By the SageLift team,
Most sizing mistakes in cold email come from working backwards from a tool's plan limits instead of forwards from a sending target. Start with how many emails you want to send per day, then build the infrastructure to carry that volume conservatively.
Step 1: Pick a daily sending target
Decide how many new conversations you want to start each day across all campaigns, including follow-ups. If you're not sure, start smaller — you can add mailboxes later.
Step 2: Convert it into mailboxes
For planning, we assume about 20 emails per mailbox per day. That keeps each mailbox's volume modest. So:
Mailboxes = ceiling(daily emails ÷ 20)
Step 3: Convert mailboxes into domains
We recommend 3 mailboxes per sending domain, which spreads volume and limits the impact if one domain runs into trouble:
Domains = ceiling(mailboxes ÷ 3)
Worked examples
| Emails / day | Mailboxes | Domains |
|---|---|---|
| 500 | 25 | 9 |
| 1,000 | 50 | 17 |
| 2,000 | 100 | 34 |
| 5,000 | 250 | 84 |
| 10,000 | 500 | 167 |
What these numbers are — and aren't
They're planning ratios, not guaranteed safe limits. Deliverability also depends on list quality, copy, authentication, warm-up and how recipients respond. Treat the ratios as a sensible starting point and adjust with real data.
Want the math done for you? Try the infrastructure calculator or the campaign volume planner.