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 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 api vs 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.