Proofmail

Checks whether a mailbox actually exists — and says "I don't know" instead of guessing when it can't.

Try: a real mailbox a dead Gmail a Yahoo address

Use it from code

curl "https://proofmail-email-verification-api.p.rapidapi.com/v1/verify?email=someone@company.com" \
  -H "x-rapidapi-key: YOUR_KEY"

Grab a key on RapidAPI — the free tier is 500 verifications a month, no card. Paid plans are priced per verification backed by a live SMTP dialogue: you pay for proof, and mailbox_verified: true is only ever said with that proof behind it. Where proof isn't possible, you get risky or unknown with a machine-readable reason — why that costs money and a weekend project doesn't cut it.