27 CFR §44.185

Every package of tobacco products shall, before removal from the factory under this subpart, have adequately imprinted thereon, or on a label securely affixed thereto, the words “Tax-exempt. For use outside U.S.” or the words “U.S. Tax-exempt. For use outside U.S.” except where a stamp, sticker, or notice, required by a foreign country or a possession of the United States, which identifies such country or possession, is so imprinted or affixed.