How many mailboxes per domain for cold email?
Updated 17 October 2026 · 5 min read
Short answer
Use 2–3 mailboxes per sending domain for cold email. Each mailbox sends about 30 (at most 50) cold emails a day, so a domain carries roughly 60–150 emails a day, and you scale by adding domains rather than stacking more mailboxes on one domain. For example, 600 emails a day needs about 20 mailboxes on 7–10 domains.
The 2–3 rule
The common practice among cold email teams is to put 2 or 3 mailboxes on each secondary domain, and to cap each mailbox at around 30 cold emails a day (50 at the very most, including follow-ups). This is a practitioner rule of thumb, not a published limit from Google or Microsoft, but it follows directly from how mailbox providers judge reputation.
Why not 10 mailboxes on one domain?
- Reputation is tracked per domain. Gmail, Yahoo and Outlook.com all look at the sending domain. Ten mailboxes on one domain at 30/day each is 300 cold emails a day from one domain, which is what one heavy sender looks like.
- Blast radius. If a domain gets flagged, every mailbox on it is affected. With 3 mailboxes per domain, one bad domain takes out 3 mailboxes, not 10.
- Bulk-sender thresholds. Google, Yahoo and Microsoft apply stricter rules to senders of 5,000+ messages a day to their users. Spreading volume across domains keeps each domain well below that level (you should still meet those rules; see bulk sender requirements).
- Easier rotation. Small units make it simple to rest a domain, warm a replacement, and keep campaigns running.
Why not 1 mailbox per domain? It works, but it doubles or triples the number of domains you have to buy, configure and renew for the same volume. 2–3 is the usual balance between cost and risk.
How many mailboxes and domains for N emails a day
Formula: mailboxes = daily emails ÷ 30 (round up), domains = mailboxes ÷ 3 (round up; use ÷ 2 for a more cautious setup). Daily emails includes follow-ups, not just first touches.
| Emails per day | Mailboxes (at 30/day) | Domains (3 per domain) | Domains (2 per domain) |
|---|---|---|---|
| 100 | 4 | 2 | 2 |
| 300 | 10 | 4 | 5 |
| 500 | 17 | 6 | 9 |
| 1,000 | 34 | 12 | 17 |
| 2,000 | 67 | 23 | 34 |
| 3,000 | 100 | 34 | 50 |
| 5,000 | 167 | 56 | 84 |
How to plan your setup
- 1
Set your daily volume
Count first emails plus follow-ups you plan to send per day across all campaigns.
- 2
Work out mailboxes
Divide by 30 and round up. If your lists are risky or your domains are new, divide by 20 instead.
- 3
Work out domains
Divide mailboxes by 3 (or 2) and round up. Add 10–20% spare domains for rotation.
- 4
Buy and configure
Buy lookalike .com domains, set SPF, DKIM and DMARC, create the mailboxes and start warmup. See the setup guide.
- 5
Review monthly
Retire or rest domains whose bounce or spam numbers worsen and replace them with warmed spares.
What it costs
Your cost is mostly mailboxes plus domains. With ReplyDak, mailboxes are from $2.69 each (Starter $35/month for 10, Growth $87 for 30, Scale $269 for 100) and domains are $13 a year each. A 30-mailbox setup on 10 domains is $87 a month plus $130 a year for domains. Run your own numbers in the cost calculator or see pricing.
Pick a mailbox count and ReplyDak spreads them across domains, sets up DNS and starts warmup automatically.
Domains, DNS, Google Workspace mailboxes and warmup. From $2.69 per mailbox ($35 for 10).
FAQ
Is there an official limit on mailboxes per domain?
No. Google Workspace and Microsoft 365 let you create many users on one domain. The 2–3 rule is a cold email best practice to limit how much volume and risk sits on any single domain.
How many domains do I need for 1,000 cold emails a day?
About 34 mailboxes at 30 emails a day each, spread across 12 domains (3 mailboxes each) or 17 domains (2 each).
Can I use 5 mailboxes per domain?
Some teams do, but it puts more volume and more mailboxes at risk on one domain. If you go above 3, lower the per-mailbox daily volume so the domain's total stays modest.
Do follow-ups count toward the daily limit?
Yes. Every message a mailbox sends counts, so plan your mailbox count on total sends including follow-ups.