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 developers picking email infrastructure
The question here is which one a backend engineer would rather operate. That means SDK depth in the language already in use, whether a retry can safely be replayed, how much the event log tells you when a customer says the email never arrived, and whether the pricing curve stays sane as volume grows.
Applied to smtp, that means weighing developer experience, SDK breadth, idempotency keys, and operating track record ahead of the rest, against password resets, receipts, and magic links shipped from application code.