`POST /api/v2/emails` queues one transactional message. Batch accepts up to 100 of the same objects. List returns metadata and `last_event`; fetch the body with Get.

- Provide exactly one of `html`, `tiptap`, or `markdown`. Markdown is rendered to HTML.
- Omit `text` to generate it from the rendered HTML. `text: ""` skips generation.
- `to` is a string or a one-address array.
- Send and batch errors: `{ name, message, statusCode }`.