What happens at your cap

SMTP queues. HTTP and REST reject. The dashboard and an admin email tell you.

Each cluster has an hourly throttle and a rolling 24-hour cap. Those numbers are the plan, or a higher 24-hour boost when one is active.

When either cap is exhausted:

• SMTP accepts the message. It waits in the queue and sends when capacity opens. Already accepted mail is not dropped.
• HTTP inject and POST /sends return 429 {error: throttled}. Keep the message in your app and retry later. Immediate retries hit the same cap.
• The account dashboard shows an amber banner on every page for that cluster, including how many messages are waiting.
• Admins get one email per hour with a link to the cluster. The mail is queued, not blocked.

Boost and change plan sit on the banner for admins when those actions are available. A boost does not change the monthly plan. REST keeps returning throttled until the hourly throttle or 24-hour cap has room again.

See Clusters and Errors.