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 developers picking email infrastructure
The question here is which one a backend engineer would rather operate. That means SDK depth in the language already in use, whether a retry can safely be replayed, how much the event log tells you when a customer says the email never arrived, and whether the pricing curve stays sane as volume grows.
Applied to transactional email, that means weighing developer experience, SDK breadth, idempotency keys, and operating track record ahead of the rest, against password resets, receipts, and magic links shipped from application code.