X-Request-Id when contacting support. Missing, expired, and foreign private resources intentionally share one 404 response shape.
Retry transient 429, 500, and 503 responses with exponential backoff and jitter. Honor Retry-After. Keep the same create-email idempotency key but generate a new network request. Do not retry a permanent client response until you correct its input, scope, or credential.
If a source or attachment object is temporarily unavailable, the API returns 503; it never returns a successful empty file.