How Many Domains Do You Need for 1,000 Mailboxes?
For 1,000 mailboxes, plan on roughly 330–500 domains, depending on whether you put 2 or 3 mailboxes on each. Most agencies land on 3 mailboxes per domain, which means about 334 domains.
But the ratio is the easy part. Running that many domains is an operations problem.
The ratio
| Mailboxes per domain | Domains for 1,000 mailboxes |
|---|---|
| 2 | 500 |
| 3 | 334 |
| 4 | 250 |
| 5 | 200 |
Fewer mailboxes per domain means a burned domain takes down fewer inboxes, at the price of more domains to buy, configure and renew. Two to three is the usual compromise.
What 1,000 mailboxes can send
At 30 emails per mailbox per day and 22 sending days a month:
- 30,000 emails per day
- ~660,000 emails per month
- ~220,000 new prospects per month with a 3-step sequence
If you don’t need that much, scale down. The Email Sending Calculator works backwards from a monthly target.
Rough monthly cost
With editable defaults of $12/year per domain and $7/month per mailbox:
- Domains: 334 × $12 ÷ 12 ≈ $334/month
- Mailboxes: 1,000 × $7 ≈ $7,000/month
Add your sending platform, warmup and lead data on top. Plug in your actual prices in the Cold Email Cost Calculator.
Choosing domains
- Use variations of the brand that a prospect would recognize:
getacme.com,tryacme.com,acmehq.com. - Prefer common TLDs like
.com, plus a few established alternatives. Avoid cheap TLDs with poor reputations. - Redirect every outreach domain to your main website so curious prospects land somewhere real.
- Never send cold email from your primary company domain.
Mixing providers
Many large setups split mailboxes between Google Workspace and Microsoft 365, and sometimes private SMTP. Spreading across providers reduces the risk of one provider’s policy change hitting everything at once. The trade-off is more admin consoles, more DNS patterns and more places for configuration to drift.
The real problem: keeping 334 domains healthy
At this scale, something is always slightly broken:
- A domain is about to expire because auto-renew failed on one card
- Someone edited DNS and removed the DKIM record
- A new tool was added and pushed SPF over 10 lookups
- A domain landed on a blocklist
- Nobody knows which client owns which mailboxes
Most teams manage this with spreadsheets and spot checks. That works for 20 domains. It doesn’t work for 334.
Free tools help with one-off checks — try the Domain Expiry Checker or the Domain Health Check — but you need continuous monitoring at this scale. That’s what FeedPipeline Monitor is for: domain and mailbox inventory per client, daily DNS checks, change detection, expiry and blocklist alerts in Slack.
Summary
- 334 domains at 3 mailboxes each is the common answer for 1,000 mailboxes.
- Expect ~660,000 emails/month capacity at 30/mailbox/day.
- The cost is mostly mailboxes, not domains.
- The hard part is operational: monitor every domain continuously or problems will find you first.