SMTP
The protocol that delivers email between mail servers.
Simple Mail Transfer Protocol is the standard for sending email. Most providers offer both an SMTP relay and an HTTP API. SMTP is convenient when migrating from a self-hosted setup; HTTP APIs are easier to debug and authenticate.
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 smtp, 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.