Which of the following is considered an unacceptable practice regarding mail security?

Opening the mail is considered an unacceptable practice regarding mail security primarily because it violates the privacy and confidentiality rights of the sender and recipient. The integrity of mail is protected under various laws and regulations, which establish that only the intended recipient or authorized individuals have the right to access the contents. When someone other than the designated recipient opens the mail, it not only breaches trust but can also lead to the exposure of sensitive information, thus compromising the security that the postal system is meant to uphold.

In contrast, the other practices mentioned—such as the proper delivery of mail, scanning mail for delivery, and secure packaging of mail—are essential components of the mail handling process. Proper delivery ensures that mail reaches its destination efficiently and safely. Scanning allows for tracking, which enhances accountability and provides customers with updates on their mail's status. Secure packaging ensures that items are protected during transit, further ensuring their safety and security. Each of these practices aligns with the overall mission of the USPS to maintain high standards of service and security for all mail.