Which center processes incoming and outgoing letters and packages for military installations?

The Official Mail Center is responsible for processing incoming and outgoing letters and packages for military installations. This center plays a crucial role in managing the official correspondence of the military, ensuring that all mail is handled in accordance with regulations and delivered efficiently to their respective recipients.

Official Mail Centers are specifically designed to handle official government mail, which includes letters and packages sent on behalf of military units. They help maintain a streamlined process for the distribution of essential communications and ensure that all mail services comply with military standards and policies, which is critical for operational effectiveness within military contexts.

While other options may also relate to mail processing in different capacities, the primary function of the Official Mail Center is to focus specifically on the correspondence of military organizations, making it the most appropriate answer for this question.