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 teams building autonomous agents
An agent stack treats email as a two-way primitive. Inbound has to arrive as structured JSON a tool call can consume, sends have to be safe to retry when a model loop repeats itself, and every action needs an audit trail. A provider without inbound parsing only solves half the problem.
Applied to smtp, that means weighing inbound parsing, idempotency keys, webhook coverage, and operating track record ahead of the rest, against inbound parsing into tool calls and model-generated sends that must not duplicate.