Email Spam Tester
Send a test message and get a spam score, 41 checks with RFC citations and a fix plan

Send emails directly from client-side JavaScript without a backend server.
EmailJS API is a free Email API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Well organized and easy to understand documentation with examples!
Details from emailjs.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.
Send a test message and get a spam score, 41 checks with RFC citations and a fix plan
Validate email syntax and MX, detect disposable/role addresses, suggest typo fixes
Validate email addresses
The EmailJS API is provided by EmailJS (emailjs.com). Well organized and easy to understand documentation with examples!
Yes. The EmailJS API is listed in the Email 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 EmailJS API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The EmailJS API supports CORS, so front-end JavaScript can call it directly.
Yes. The EmailJS API is served over HTTPS, so it's safe to use in production.