Which of the following is NOT one of the categories of international mail?

In the context of international mail, categories are defined based on the service features and intended use of the mail items. GXG (Global Express Guaranteed), PMEI (Priority Mail Express International), and PMI (Priority Mail International) are all recognized categories that provide specific international mailing services with varying levels of speed and tracking.

GXG offers fast delivery with money-back guarantees, while PMEI and PMI are options under Priority Mail that ensure timely delivery and include certain tracking features.

The category labeled as "EXPRESS" is not specifically a recognized category of international mail by USPS standards. Instead, it may refer to express services in a more general sense without the unique identifiers and service distinctions that the other three options provide. Therefore, it's accurate to identify it as not being one of the established categories of international mail.