Email verification API.
Kickbox API is a free Email API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from open.kickbox.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.
Free email validation: MX check, disposable detection, syntax
The Kickbox API is provided by Powered by Kickbox (open.kickbox.com).
Yes. The Kickbox 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.
No. The Kickbox API requires no authentication, so you can start making requests without signing up.
Yes. The Kickbox API supports CORS, so front-end JavaScript can call it directly.
Yes. The Kickbox API is served over HTTPS, so it's safe to use in production.