Specialized queries to the mail service, such as VRFY or EXPN in SMTP (Simple Mail Transfer Protocol), can determine which addresses exist by responding with relevant user information. Using a directory on a public site may reveal generic contact details, but it does not confirm valid addresses at the mail server. Attempting to gather ephemeral keys from leaked code repositories does not directly verify active addresses. Capturing traffic on a local network does not determine which accounts reside on a mail server.
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