Create a temporary test inbox

Step 1 of a mail test. No request body is required. Returns top-level emailId, emailAddress and expiresAt (Unix milliseconds); the session expires after seven days. Send an email to emailAddress, then poll check with emailId. Rate limit: 10 requests per minute per IP, plus an in-memory 50-per-hour check and a cap of 50 stored sessions created in the previous hour. The hourly in-memory check shares the legacy default limiter bucket. RateLimit headers describe the 10-per-minute wrapper; Retry-After is present only when that wrapper refuses the request.