Idempotency and retries

A request can time out after Sendery has accepted your email. Sending it again could leave the recipient with two copies.

An idempotency key identifies one email. Retrying the same request with the same key returns the original email’s ID and status instead of sending another copy.

Include an Idempotency-Key header when sending an email.

• Create a unique key for each new email.
• Keep the key and the entire request unchanged on retries, including background jobs.
• Use a new key when you intend to send another email.

Changing the request while reusing its key returns 409 idempotency_conflict.

Retry timeouts, temporary service failures, and rate_limited errors after a delay. If the response includes Retry-After, wait the specified number of seconds.

Fix authentication, template, or billing errors before trying again.