Marketing email can be sent and mail templates made in MJML or HTML can be sent using API.
mailjet API is a free Business API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Design, deliver, and automate marketing & transactional emails together with built-in deliverability, GDPR compliance, and developer-ready email APIs
Details from mailjet.com.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Company lookup
The mailjet API is provided by Sinch Mailjet (mailjet.com). Design, deliver, and automate marketing & transactional emails together with built-in deliverability, GDPR compliance, and developer-ready email APIs
Yes. The mailjet API is listed in the Business category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The mailjet API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the mailjet API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The mailjet API is served over HTTPS, so it's safe to use in production.