POST /emails to send one message, or POST /emails/batch for up to 100 messages. Batch requests do not support attachments. The API reference lists the supported fields and limits.
Idempotency
Use anIdempotency-Key header for sends and inbox creation. Store the key with your application job so retries reuse it with the same payload. Keys are retained for 24 hours. Do not use one key for different messages or generate a fresh key after an ambiguous timeout.
A batch can partially dispatch if infrastructure fails after validation. Investigate its outcome before issuing a new batch with a different key.
Scheduling
Setscheduled_at to an ISO timestamp or a supported relative expression such as in 1 hour, up to 30 days ahead. Only scheduled messages can be rescheduled or cancelled. Once dispatch has begun, cancellation is not a recall mechanism.
Delivery status
An accepted send returns{ "id": "…" }. Read the message or its events to track progress. Webhooks provide delivery notifications without polling.
A delivered event means the receiving server accepted the message. It does not prove inbox placement or that someone read it. Open and click tracking can be affected by privacy features and automated scanners.