How many mailboxes per domain and domains per campaign (best practice)

Draft — written from product facts + domain knowledge; UI labels to be verified before publishing.

Cold email scales horizontally: many mailboxes sending a little each, spread across many domains, rather than one mailbox sending a lot. This article gives the numbers to plan your infrastructure and shows where to set them in OutreachFox.

The numbers

Sizing example

You want to contact 5,000 prospects over 4 weeks with a 3-step sequence (≈15,000 sends, ≈750/day on working days).

1. 750 ÷ 40 per mailbox ≈ 19 mailboxes.
2. 19 ÷ 3 per domain ≈ 7 domains.
3. Buy 7 domains and 21 mailboxes through Icemail; warm up for 2–3 weeks; then launch.

Steps: apply the limits in OutreachFox

1. Go to Mailbox.
2. Select all mailboxes in the batch (use the select-all checkbox at the top).
3. In the bulk actions bar, set campaign emails per day to 20 for newly warmed mailboxes; raise to 40–50 after two weeks of clean sending.
4. Open the campaign → Edit Campaign → Settings → Show Advanced Configurations.
5. Set Daily new contacts limit so total daily sends across all steps stays under (mailboxes × per-mailbox limit).
6. Set Pause campaign when bounce rate reaches to 3%.
7. Under Sender Accounts, add every mailbox from every domain reserved for this campaign.
8. Click Save, then Resume Campaign or Launch Campaign.

What you see afterwards

The campaign's daily send volume is capped by the sum of its sender accounts' limits, and OutreachFox rotates sends across mailboxes and domains.

Tips

• Keep warm-up running on mailboxes that are in campaigns, at a reduced rate (around 20/day). Turning it off after launch is the most common reason health scores decay.
• Mix providers (Google + Microsoft) across domains and enable Auto match leads so recipients get mail from a matching ESP.
• Do not run more than one campaign at full volume from the same small set of mailboxes; the per-mailbox limit is global, so campaigns share it.
• Give every mailbox a real person's name and a signature. Five firstname@ accounts on a domain is normal; five sales1@…sales5@ is not.
• Retire and replace, rather than push, any mailbox whose health score stays below 70 for a week.

Troubleshooting

• Campaign sends fewer than expected. The sender accounts' combined daily limit is lower than the campaign's daily new contacts limit plus follow-ups. Add mailboxes or raise limits gradually.
• One domain has much higher bounce/spam than the rest. Pause its mailboxes, check blacklists and DNS, and let the rest of the campaign continue.
• Provider suspended a mailbox. You exceeded provider limits (Google Workspace hard-caps at ~2,000/day but flags far lower for cold mail). Stay in the 30–50 range.

Related articles

• Buy mailboxes through Icemail (Google, Microsoft, IMAP/SMTP, Azure)
• Warmup best practices: how long, how fast, and when to launch
• Set campaign sending volume and daily limits
• Enable bounce protection (pause at X% bounce)
• Enable Auto match ESP
• Deliverability fundamentals for cold email