Use when wiring server code to send transactional or bulk email via Resend, SendGrid, or Postmark: a provider-agnostic sendEmail() seam, idempotent retries, 100-cap batches with partial failures, transactional-vs-broadcast streams, bounce webhooks feeding a suppression list. NOT SPF/DKIM/DMARC inbox reputation (that is `email-deliverability`).
