The U.S. Postal Service and the association that represents envelope makers say their glue does not contain harmful gluten. In fact, there are only a few envelope glue makers in the U.S. and the largest one makes its adhesive from corn. More than 98 percent of all stamps sold by the U.S. Postal Service are self adhesive and do not require licking. The other 2 percent do not contain gluten in the glue.