Inbox placement
Whether delivered email lands in the inbox instead of spam.
Delivery only means the receiving server accepted the message. Inbox placement asks where the message landed after filtering. Seed-list tests and panel data can estimate placement, but no test perfectly represents every recipient inbox.
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 inbox placement, 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.