Two numbers hide in this question. The provider limit is the hard cap Gmail, Google Workspace or Microsoft 365 enforces on a mailbox's daily sending. The safe cold volume is how much unsolicited mail the domain can send before receivers start filtering it. Send to the first number and you find the second the hard way, usually with a placement test showing spam at one provider first.
The provider caps, in their words
Know these so you never mistake them for guidance. Each row is the provider's own documentation with the date it was read; the linked page covers recipients, rate limits, trial restrictions and what happens when you exceed it.
| Provider | Daily cap as documented | Source date |
|---|---|---|
| Google Workspace | 2,000 per user per day (1,500/day via mail merge / multi-send); trial accounts 500/day | |
| Gmail | 500 messages/day | provider page |
| Microsoft 365 | Recipient rate limit 10,000 recipients per mailbox per 24h | |
| Outlook.com | 5,000 recipients/day; 1,000/day to first-time (non-relationship) recipients | |
| Zoho Mail | No fixed daily cap published; external 50–500 per hour (dynamic, reputation-based); internal 1,000/hour | provider page |
| Yahoo Mail | Not disclosed (anti-spam limits are undisclosed) | provider page |
| iCloud Mail | 1,000 messages/day |
Safe cold volume by domain age
These are MailStrike's working figures, not measurements, and they are governed by placement rather than by the calendar.
- Domain under 30 days: no cold outreach until the mailbox is at full volume, which on the default ramp is day 21. Then 20 to 30 a day per mailbox.
- Domain 31 to 90 days, warmed: 20 to 30 a day per mailbox as the start, scaled as placement holds on a test. The domain has history now; it still has little.
- Established, warmed domain: scale from the same start as placement allows, and add mailboxes rather than pushing one. Warming continues at a maintenance level of about 15 to 25 a day.
- Any domain: back off the day placement slips or complaints rise. The safe number is the one that keeps the mail in the inbox.
How many mailboxes and domains you need
Total volume is a fleet question. Put in the daily total you want and the calculator gives the mailboxes and domains, each staying in its own range.
Arithmetic only. The per-mailbox figure is the outreach volume MailStrike advises once the ramp is done (20 to 30 a day, then scale with placement); three mailboxes a domain is common practice, not a measurement. Every mailbox in the total needs its own warming.
- Every mailbox warms. A mailbox added for capacity needs its own 21-day ramp before it sends; the multi-domain warming guide covers running a fleet in parallel.
- Keep the risk isolated. A few mailboxes per domain means one domain's problem does not take the whole operation with it.
- Check the caps against the ramp. Each provider's limits page shows where its cap sits against a warmup ramp, with the calculator; the answer is always that the cap is not the constraint.
Why overshooting hurts
Volume that outpaces reputation looks like a spam run. Receivers expect sending to grow in proportion to established trust, so a spike from a thin-reputation domain trips defensive filtering; the bounce codes providers return for it (Google's 421 4.7.28 “unusual rate of unsolicited mail”, 550 5.7.1 rate limited) are in the bounce code decoder. High volume also magnifies list problems: a small complaint or bounce rate on a big send produces enough negative signal to dent the domain, and the reputation checker shows the public side of that damage. Lower, steadier volume from a warmed domain reliably beats the blast.
The short version
Provider caps are technical ceilings, documented above. For cold outreach a mailbox sends nothing until the dashboard says it is safe to send from, then 20 to 30 a day, then more as placement holds, with warming continuing underneath. To send more in total, add warmed mailboxes and domains; the calculator gives the count.