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 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 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.