325 DOT Hazardous Materials Warning Labels and Markings ... 325.3 Mailable Warning Labels. The warning labels shown in Exhibit 325.3 a, Exhibit 325.3 b, and Exhibit 325.4 may appear only on mailpieces containing mailable hazardous materials that require use of the label under Postal Service requirements. Division 5.1, 5.2, Class 8 and Class 9 labels are only permitted when used in conjunction with a ...