Idempotency

Retry a send after a timeout or crash without risking a duplicate.

Send an Idempotency-Key header with up to 255 printable characters. Use a new key for each logical send and reuse it when retrying that same send.

curl -X POST 'https://api.sendsend.email/v1/emails' \
  -H "Authorization: Bearer $SENDSEND_API_KEY" \
  -H "Idempotency-Key: order-1234-receipt" \
  -H 'Content-Type: application/json' \
  -d '{ ... }'

You send	You get
A new key	A new message.
The same key and the same body	The original message, with an `Idempotent-Replayed: true` header.
The same key and a different body	`409 idempotency_conflict`.

Keys are remembered for 24 hours. A good key is something from your own data, like an order id plus the purpose.