Rate limits

Two kinds of limit apply to every send:

Send limits cap how fast an inbox sends, to anyone.
Cold outreach cap limits how many new recipients you email per UTC day. Starts low and grows as you build history.

Replies and follow-ups only meet the first. Cold outreach meets both.

Send limits

Limit	Value	Scope
Burst	10/minute	Per inbox
Daily	200/day	Per inbox
Inbox creation	100/day	Per account

Over one of these, the API returns 429 with a Retry-After header:

{
  "error": "rate_limit_exceeded",
  "message": "Max 10 sends per minute per inbox"
}

Cold outreach cap

A recipient is new the first time an inbox emails an address that has never written to it. Each To/Cc address counts once; Bcc, your own inboxes, and anyone who has already emailed or been emailed by that inbox don’t count. Warmth is per inbox, so a contact known to inbox A is still new to inbox B.

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Three caps apply at once. The lowest one you reach is the one that blocks.

Scope	Warm-up (first 7 days or 100 new recipients)	After
Inbox	20/day	no cold cap (200/day send limit still applies)
Custom domain	tier schedule below	tier schedule below
Account	150/day	2× your trailing 7-day average, 150 to 500

Custom domain tiers. Each domain climbs on its own. Both columns are counted from the day you added the domain to OpenMail; how long you’ve owned it doesn’t matter. Inboxes on the shared omail.sh domain have no domain cap.

Tier	Days since domain added	New recipients messaged so far	New recipients/day
0	0+	0+	20
1	7+	50+	50
2	14+	150+	100
3	28+	400+	your account cap

To grow your caps

Send to new recipients most days. The account cap follows your trailing average, and a domain’s tier needs volume as well as age.
Keep bounces under 5% and complaints under 0.1%. Anything worse throttles the inbox, and at 10% bounces or 0.3% complaints suspends it; see Reputation.
Get replies. Cold threads that get a human reply are what let the account cap keep doubling.

Need volume before the history exists? Contact us about a trusted volume for a vetted use case.

Scope	First 30 days	After 30 days
Inbox	20/day	200/day
Account	30/day	100/day

When you get a 429

{
  "error": "cold_outreach_limit",
  "message": "This sending domain has reached today's limit of new recipients. The limit rises as the domain ages and builds a clean sending history. Try again after midnight UTC.",
  "scope": "domain",
  "limit": 20,
  "resetsAt": "2026-09-30T00:00:00.000Z"
}

Read the scope

inbox, domain or customer (your account). It tells you which cap you hit, so you know whether another inbox or domain would help. On a domain block, another inbox on the same domain won’t.

Pause cold sends until resetsAt

Caps reset at midnight UTC. Retrying earlier fails the same way. There’s no endpoint for today’s remaining allowance yet, so treat the 429 as the signal.

Keep replying

The rejected send didn’t go out and didn’t count. Replies and follow-ups to existing contacts are unaffected, so your agents can keep working threads.

Reputation

Every inbox has a reputation state: good, watch, throttled or suspended. It’s computed from the inbox’s permanent bounces and spam complaints over the trailing 7 days. Keep bounces under 5% and complaints under 0.1% and the inbox stays in good, with full caps.

Thresholds

Signal	`watch`	`throttled`	`suspended`
Bounce rate	Any bounce	5%	10%
Complaint rate	Any complaint	0.1%	0.3%

What each state does

State	Effect	How it clears
`watch`	Caps stop growing; a custom domain drops one tier	On its own, as the rate falls
`throttled`	Daily send limit cut to 25%; cold caps halved	On its own, as the rate falls
`suspended`	Every send from the inbox refused	After review by OpenMail; the `inbox.reactivated` webhook fires

If an inbox is suspended

Fix what caused the bounces or complaints: validate the list, check DNS, drop the source.
Contact us for review.
Wait for the inbox.reactivated webhook before sending again.

For how to send so you never meet these limits, read Email deliverability.

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Security & privacy

Send limits

Cold outreach cap

When you get a 429

Reputation

Thresholds

What each state does

If an inbox is suspended

​Send limits

​Cold outreach cap

​When you get a 429

​Reputation

​Thresholds

​What each state does

​If an inbox is suspended

Send limits

Cold outreach cap

When you get a 429

Reputation

Thresholds

What each state does

If an inbox is suspended