Which of the following is NOT an issue related to the security of the mail?

Sending is not considered an issue related to the security of the mail because it refers to the process of dispatching postal items rather than a vulnerability or risk associated with the mail system itself. The act of sending mail is a fundamental function of the postal service and is intended to facilitate communication and delivery.

In contrast, theft refers to items being unlawfully taken from the mail, destruction involves damage to mail while in transit, and opening pertains to unauthorized access to mail content. Each of these reflects legitimate concerns regarding the integrity and safety of the mail that could compromise the privacy and delivery of items sent through the postal system. Thus, while sending is central to the USPS's operations, it does not fall within the scope of security issues that the postal system faces.