I _think_ the idea is just to use React as a templating language to write HTML emails, rather than dealing with the quirks of email HTML directly; it's not about sending emails from the frontend.
The selling point is that this allows leveraging existing React knowledge, as per the founder's comment upthread.
There's also react-mjml, a React wrapper around it. Works great, altho it's easy to mix up React props with mjml props. Just requires devs to actually read the documentation