Sandbox mode
A starter mode that limits sending to verified addresses.
Some providers (most notably AWS SES) start every account in sandbox mode: you can only send to verified addresses until you request and receive production access. Plan for the approval lead time when launching.
reading this as teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to sandbox mode, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.