API vs SMTP
When to use HTTP API versus SMTP relay.
HTTP APIs return structured errors, support idempotency keys, and integrate with serverless runtimes. SMTP is universal and works with any language. APIs are usually preferred for new code; SMTP is preferred when migrating an existing app or using libraries that already speak SMTP (like Nodemailer or ActionMailer).
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 api vs 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.