Which method is NOT approved for mailing classified information?

Curbside mailboxes are not approved for mailing classified information because they do not provide the necessary security and tracking measures required for such sensitive materials. Classified information must be safeguarded against unauthorized access and tampering, and curbside mailboxes lack the physical security, control, and reliability needed to ensure that classified documents reach their intended destination safely.

In contrast, registered mail and secure courier services offer enhanced security features such as tracking, delivery confirmation, and handling by trusted personnel. FedEx overnight service can also be utilized for classified materials as it provides secure shipping options that comply with regulations governing the transport of sensitive information. Thus, the use of curbside mailboxes fails to meet the stringent requirements for handling classified materials, making them an inappropriate method for mailing such information.