Rejection code 256 Not Authorized means driver is a FAST Driver.

A commercial clearance program known as Free and Secure Trade (FAST) allows low-risk shipments from Canada and Mexico to enter the United States. Designed after 9/11, the trusted traveler/trusted shipper program expedites the process for commercial carriers who have completed background checks and are eligible. 

The FAST program is open to truck drivers in the United States, Canada, and Mexico.