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 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 api vs smtp, 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.