Which portions of a classified email must be marked?

The correct answer is that the email subject line must be marked. In the context of handling classified information, the subject line is considered a critical part of the overall context of the email. It provides an immediate indication of the content's sensitivity, helping recipients to understand the classification level before even opening the email.

Marking the subject line serves as an important step to ensure that individuals handling the email are aware of the classified nature of the information contained within. This aligns with protocols for safeguarding classified materials, as it helps maintain visibility and awareness of the potential risks associated with the information being communicated.

While other parts of the email also contribute to the overall communication—such as the sender's name and email address, greetings, and the content of the message itself—these elements do not carry the same level of immediate significance in terms of classification marking. The subject line stands out as a key area that warrants explicit marking to protect the classified information effectively.