Open-source intelligence (OSINT) refers to the collection and analysis of information that is gathered from public, or open, sources. In the context of confirming the legitimacy of an email, OSINT could involve checking online databases, public records, or using search engines to find information related to the email's domain, the sender, links included in the email, or file hashes of attachments. This method can help determine if the email is part of a known phishing campaign or if it has been flagged by others as suspicious.