What happens at your cap
Every send path refuses the message. The dashboard and an admin email tell you.
Each cluster has an hourly cap and a rolling 24-hour cap. This page covers what each send path does when a cap is full, how PostShiba tells you, and how to raise it.
The two caps
| Cap | API field | Window |
|---|---|---|
| Hourly throttle | emails_per_hour |
The current hour |
| 24-hour cap | emails_per_day |
The last 24 hours, rolling |
Both come from the cluster's plan. An active capacity boost raises them for its 24 hours. Use the Hour and 24h toggle on the cluster page to see how much of each is used.
What each send path does
| Send path | When either cap is full |
|---|---|
| SMTP | Returns 451 4.7.0 hourly send limit reached, or 451 4.7.0 daily send limit reached at the 24-hour cap, and does not keep the message. Mail servers retry a 451 on their own. Most app libraries raise an error, so retry once the cap has room. |
| HTTP inject | Returns 429 hourly send limit reached, or 429 daily send limit reached at the 24-hour cap. |
REST POST /api/v1/emails and POST /sends |
Returns 429 throttled. |
A REST rejection looks like this.
response.json
1{
2 "error": "throttled",
3 "field": "base",
4 "message": "This cluster hit its hourly send limit"
5}
When the 24-hour cap is the one that is full, the message reads This cluster hit its 24-hour send limit. Retrying right away hits the same cap. Other error strings are on Errors.
How you find out
- An amber banner runs across the top of every dashboard page for that cluster. It names the cluster, the hourly cap, and how many messages are scheduled and ready.
- Team Administrators see Change plan on the banner. When capacity boosts are enabled, they also see Boost.
- Team Administrators and the billing contact get an email with a link to the cluster, at most once an hour.
Raise the cap
- Select Resize on the cluster page to move to a larger plan. See Clusters.
- If capacity boosts are enabled for your account, buy a 24-hour boost. A boost does not change the monthly plan. See How do I buy a capacity boost?.