Transactional email
One-to-one email triggered by user action.
Transactional email is sent in response to a user-initiated action: password reset, receipt, magic link, account verification. Volume is unpredictable, latency matters, and missed delivery is a product failure. It contrasts with marketing email, which is one-to-many and sent on the senders schedule.
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 transactional email, 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.