Bulk email verification with full SMTP server responses.
uchecker API is a free Email API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Details from api.uchecker.net.

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.
Verifies that a given email is real
Email for AI agents: send email, receive replies as threads, with per-key send limits and approvals
A service that provides solutions relating to marketing and/or transactional email and/or SMS
The uchecker API is provided by UChecker API (api.uchecker.net).
Yes. The uchecker 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 uchecker API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the uchecker API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The uchecker API is served over HTTPS, so it's safe to use in production.