If you want to know the "how?" as I did - code is here, specifically https://github.com/reacherhq/check-if-email-exists/blob/master/core/src/smtp/mod.rs#L193 ht

If you want to know the "how?" as I did - code is here, specifically https://github.com/reacherhq/check-if-email-exists/blob/master/core/src/smtp/mod.rs#L193 https://github.com/reacherhq/check-if-email-exists/blob/mast... tl;dr, According to RFC 5321, `RCPT TO` command succeeds with 250 and 251. So email is valid if you get to this part of the protocol and receive the response.