How many mailboxes do you need?
Set your daily sending target and see the mailboxes, sending domains and warmup runway it requires — with the arithmetic shown, so you can argue with it.
Sending posture
How many cold emails each mailbox sends per day. This is the assumption that drives everything else.
Spreading mailboxes across domains limits the damage when one gets burned. Three is a common choice.
Your setup
17
mailboxes
6
sending domains
To send 500 emails a day at 30 per mailbox, you need 17 mailboxes — about 11,000 emails a month across ~22 working days.
The arithmetic
mailboxes = 500 ÷ 30 = 17 domains = 17 ÷ 3 = 6
Warmup ramp
New domains cannot start at full volume. A typical ramp to 500/day:
- Week 1~75/day
- Week 2~175/day
- Week 3~325/day
- Week 4+~500/day
These are planning conventions, not measured limits — no provider publishes a hard safe number, and the real ceiling depends on domain age, reputation and list quality. Treat the output as a starting point and adjust the posture to match what your own data shows. Buying the mailboxes is only step one; they still need SPF, DKIM and DMARC configured.
Why volume is a mailbox problem
The instinct is to send more from one inbox. That is the fastest way to burn a domain. Mailbox providers judge sending behaviour per account and per domain, so cold outreach at volume works by spreading sends thinly across many mailboxes rather than pushing any single one hard.
This is also why per-mailbox pricing is the thing to check before buying a platform. A tool charging per sending account punishes the exact architecture that makes cold email work, which is why the platforms built for outbound include unlimited mailboxes.
Where the numbers come from
Honestly: convention, not measurement. No provider publishes a hard limit on “safe” cold sends per mailbox per day, because there isn't one — the ceiling moves with domain age, sender reputation, list quality and how people respond. The 20 / 30 / 50 presets reflect what experienced senders commonly plan around, and that is exactly why they are editable inputs here rather than a fixed answer.
We would rather show you the formula and let you set the assumption than assert a number we cannot substantiate. If your own data says 40 works on your domains, use 40.
Three rules the calculator assumes
- 1. Never send cold from your primary domain. Use lookalike sending domains. If one gets burned, your actual company email keeps working.
- 2. Split mailboxes across domains. Concentrating twenty mailboxes on one domain means one reputation problem takes out all twenty.
- 3. Warm up before you ramp. A new domain sending at full volume on day one is the single most reliable way to land in spam.
Once the mailboxes exist they still need authentication — check yours with the SPF/DKIM/DMARC checker. For the platform that manages warmup and rotation, the tool finder matches you in five questions.