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POST /v1/emails requires an Idempotency-Key header. Generate a unique, unpredictable value for each logical create operation and store it until the outcome is certain.
Keys are scoped to the authenticated account and operation:
  • The first successful request returns 201 and creates one reservation.
  • Replaying the same key with an equivalent body returns the original resource, sets Idempotency-Replayed: true, and is not charged again.
  • Reusing the key with a different body returns 409.
  • A transient network or 5xx result should be retried with the same key.
Do not use a timestamp alone when multiple workers can create concurrently, and do not generate a new key merely because the original response was lost. Idempotency protects API-level retries; it is not a replacement for storing your own workflow state.
Last modified on August 2, 2026