During an incident, a security analyst discovers a large amount of PII has been emailed externally from an employee to a public email address. The analyst finds that the external email is the employee

When a significant data breach, especially involving PII, occurs through an employee's mailbox, placing a legal hold is the most critical first step. This action ensures that all relevant data in the mailbox is preserved immediately, preventing accidental or intentional deletion, which is crucial for forensic investigation, compliance, and potential legal proceedings.

Enabling filtering on the web proxy addresses future outgoing traffic, but it does not address the already occurred data exfiltration or preserve the existing evidence related to the incident.

Disabling public email access with a Cloud Access Security Broker (CASB) would prevent future similar incidents but does not address the immediate need to preserve evidence of the breach that already happened.

Configuring a deny rule on the firewall focuses on network-level prevention of future outbound connections, which is too broad or too late for the immediate concern of preserving evidence from a specific internal data exfiltration event.