Email leakage is the exposure of email records, such as message bodies, headers, addresses, or mailbox metadata, to an unauthorized party. It can happen through a breach, an extortion dump, a misconfigured cloud mailbox, or an insider copy of data. Even a small sample of messages can reveal naming patterns, internal contacts, vendor relationships, and active business processes.
In cyber security, email leakage matters because it gives attackers material for phishing, impersonation, and business email compromise. Stolen messages can be reused to craft believable lures, answer security questions, or impersonate real employees in follow-on fraud. Defenders treat reported leakage as a lead to verify by checking mailbox access logs, unusual forwarding rules, and exposed accounts, then rotating credentials and warning users about targeted social engineering.