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 early-stage startups
At an early stage the deciding factors are what the free tier actually covers, what the first paid invoice looks like, and how much of a founding engineer's week the setup costs. Feature depth that only matters at a million sends a month is noise until it is not.
Applied to transactional email, that means weighing free tier, best published rate per 1,000, entry price, and developer experience ahead of the rest, against a waitlist, a launch announcement, onboarding email, and the first transactional sends.