What must be marked on an email that contains secret information?

To ensure the proper handling and protection of secret information contained in an email, it is essential to include both banner markings and a classification authority block. Banner markings are critical because they immediately signify the classification level of the information, alerting recipients to the sensitivity of the content. This preemptive warning helps to control access and handling of the email's contents appropriately.

The classification authority block provides the source or authority responsible for the classification decision, which is important for accountability and adherence to regulatory standards governing classified information. This block helps recipients understand the origin of the classification and the rationale behind it, reinforcing the need for caution in handling the information.

Without these markings, the recipients may be unaware of the secret status of the email, potentially leading to unauthorized disclosure or mishandling of sensitive information. Therefore, including the proper markings is crucial for maintaining the integrity and security of classified data in any format, including emails.