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Email sends, replies, SMS sends, number provisioning, and number release require an Idempotency-Key header. Use one unique key per logical action and save it with the request.
Retry an identical request with its original key. Reusing a key with changed input returns a conflict. Attachment bytes are part of the input. A replay returns the original operation instead of executing it again. Inbox creation is different: repeat the same body and credential without an idempotency key.

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Poll the outcome

A send or number operation returns an ID and statusUrl. Read that URL using the same credential and workspace, or call GET /v1/operations/{id}. HTTP 202 indicates a pending or unknown operation. First accepted email and SMS sends return 201. A terminal replay returns 200, which can include a failed operation: inspect status, not just the HTTP code. Network failures and timeouts do not prove that an action failed. If you lose a response, retry the same request with the same key to recover its result. Neither SDK retries automatically.