Divide your planned daily cold emails by 30 to 50 to get the number of mailboxes. Put 2 or 3 mailboxes on each domain. A team sending 300 cold emails a day needs 6 to 10 mailboxes on 2 to 5 domains, plus one spare domain warming up. Reputation sets this limit, because provider caps sit far higher.
How do you work out the number for your team?
Start from contacts, then convert to daily sends.
- Set the number of new contacts you want to reach each week.
- Multiply by the number of emails in your sequence. Instantly recommends 4 to 7 touchpoints (Instantly, January 2026).
- Divide by 5 working days. This gives your daily send volume.
- Divide by your per-mailbox limit. Instantly's help centre says 30 to 50 emails a day from each account (Instantly Help Center, accessed October 2026). Use 30 while the mailboxes are new.
- Divide the mailboxes by 2 or 3 to get domains. Smartlead recommends 2 to 3 mailboxes per domain and Instantly says a maximum of 3 to 5 (Smartlead, 8 October 2026).
- Add one domain that warms up while the others send, so a domain with a bounce problem can rest without stopping the campaign.
Here is that arithmetic at three volumes, at 30 emails per mailbox and 3 mailboxes per domain:
| New contacts per week | Steps per sequence | Cold emails per working day | Mailboxes | Sending domains | With one spare |
|---|---|---|---|---|---|
| 75 | 4 | 60 | 2 | 1 | 2 |
| 250 | 4 | 200 | 7 | 3 | 4 |
| 1,250 | 4 | 1,000 | 34 | 12 | 13 |
Smartlead's own table puts 1,000 emails a day at 6 to 8 domains, a looser ratio than ours above (Smartlead, 8 October 2026).
The first row is the important one for a founder starting out. Even 60 emails a day needs two domains, because one domain gives you nowhere to send from when it has a bad week.
Why not send more from each mailbox?
The provider limits are written for normal business email. Cold email to strangers draws spam reports at a far higher rate per message, so the safe level sits well below them.
| Limit | Google Workspace | Microsoft 365 (Exchange Online) |
|---|---|---|
| Messages or recipients per mailbox per day | 2,000 messages; 10,000 total recipients | 10,000 recipients |
| External or unique recipients per day | 3,000 | Set per tenant by licence count |
| Trial accounts | 500 messages, 500 external recipients | 5,000 external recipients per tenant |
| Rate per minute | Not stated | 30 messages |
Sources: Google Workspace Admin Help, accessed October 2026; Microsoft Learn, updated September 2026. Microsoft also caps mail from default onmicrosoft.com domains at 100 external recipients per organisation per day, so connect your own domain before you send anything.
The limit that matters is the spam rate. Gmail's sender guidelines tell senders to keep the rate in Postmaster Tools below 0.10% and to avoid reaching 0.30% (Google, accessed October 2026). At 0.30%, about three spam reports per 1,000 emails are enough to put a domain over the line. Spreading volume across more mailboxes keeps each domain's count of reports low, and keeps a bad day on one domain from reaching the others.
How long before a new domain can send?
Two to four weeks. Instantly says to warm new accounts for at least 2 weeks before a campaign, and Smartlead says two to four weeks minimum (Instantly Help Center; Smartlead, 8 October 2026). Gmail's guidelines give the same direction: start with low volume and increase it slowly, with no sudden spikes (Google, accessed October 2026).
This sets your lead time. If you plan to start outbound in November, buy and configure the domains in October. If you plan to double volume in the first quarter, add the new domains a month before the extra contacts arrive. A team that buys domains the week it wants to send has to wait or send from cold mailboxes.
Set up each domain with SPF, DKIM and DMARC before warm-up starts. Gmail requires SPF or DKIM from all senders, and SPF, DKIM and DMARC from senders of 5,000 or more messages a day to personal Gmail accounts (Google, accessed October 2026).
Does list quality change how many mailboxes you need?
Yes. Every bounce counts against the domain that sent it, so a dirty list uses up reputation that a clean list would leave in place.
Our own numbers show the gap. On the first 600 cold emails of one campaign, a list verified the week before sending bounced at 0%. Lists verified about 10 weeks earlier bounced at 3.7% to 5.2% (our data, October 2026; small samples of 43 to 219 sends per segment). When we re-verified 520 contacts marked deliverable about 10 weeks earlier, 23% came back invalid (our data, October 2026). A team that skips the re-check sends nearly a quarter of its volume to dead addresses, and adding mailboxes only spreads that damage across more domains.
Re-verify any list last checked more than 7 days before sending. We cover the cadence in how fast verified B2B email lists decay.
Should you send cold email from your main company domain?
No. Use separate domains that clearly belong to your brand, such as your name with a different ending or a short prefix. Smartlead advises against cold email from the primary business domain (Smartlead, 8 October 2026).
Your main domain carries invoices, customer support, password resets and investor updates. If cold email damages its reputation, all of that mail starts landing in spam. You can retire a damaged secondary domain and warm a replacement in two to four weeks.
Point each secondary domain at your real website, so a buyer who types it in reaches you. Use real names on the mailboxes and a signature that matches your website.
What should you decide before you buy anything?
Answer these in order, and the domain count follows:
- How many new contacts per week can you research and verify to a good standard?
- How many steps are in the sequence, and over how many days?
- What daily send does that produce, at 30 per mailbox for new accounts?
- How many domains at 2 or 3 mailboxes each, plus one spare?
- Are SPF, DKIM and DMARC set on every domain before warm-up starts?
- Does the warm-up end at least two weeks before the first campaign?
- Is every list re-verified within 7 days of its first send?
If the first answer is small, keep the infrastructure small and put the time into list quality.
To have the sending plan sized against your target accounts, start with The Diagnostic at $2,500.