Validate email addresses, phone numbers, VAT numbers and domain names.
Cloudmersive Validate 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.
Details from cloudmersive.com.

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The Cloudmersive Validate API is provided by Cloudmersive (cloudmersive.com).
Yes. The Cloudmersive Validate 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 Cloudmersive Validate API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Cloudmersive Validate API supports CORS, so front-end JavaScript can call it directly.
Yes. The Cloudmersive Validate API is served over HTTPS, so it's safe to use in production.