What is the highest security classification for material authorized to be transmitted via US Postal Service first class mail?

The highest security classification for material authorized to be transmitted via US Postal Service first class mail is Confidential. This classification is specifically designed for information that, while sensitive, does not require the same rigor of handling as Secret or Top Secret information, which have more stringent transmission protocols.

Materials classified as Confidential can be shared under first class mail, provided that they are packaged appropriately to safeguard the content and prevent unauthorized access during transit. In contrast, Secret and Top Secret materials demand higher levels of security, typically necessitating more secure means of delivery, such as secure courier services or designated government channels. Unclassified material, on the other hand, does not require special handling and can be sent through standard mailing processes without the security concerns tied to classified information.