Pass the optional Idempotency-Key header to make the request safe to retry. The key can be up to 256 characters. The server stores the canonical request body and behaves as follows:

• Same key + same request body → returns the original emailId with 200 OK without re-sending.
• Same key + different request body → returns 409 Conflict with code: NOT_UNIQUE so you can detect the mismatch.
• Same key while another request is still being processed → returns 409 Conflict; retry after a short delay or once the first request completes.

Entries expire after 24 hours. Use a unique key per logical send (for example, an order or signup ID).