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 SaaS product teams
A SaaS product sends on behalf of its customers, so the deciding factors are tenant isolation, whether suppressions can be scoped rather than global, and whether support can answer why one specific account did not receive one specific email. Lifecycle automation and transactional sending usually need to coexist.
Applied to api vs smtp, that means weighing multi-tenant support, event stream, the suppressions API, and operating track record ahead of the rest, against trial expiry notices, dunning, onboarding sequences, and per-tenant transactional mail.