Postage stamp glue is kosher in Israel because the country's population is mostly Jewish. "Kosher" is a term that refers to the dietary laws of the Jewish religion concerning how foods — especially meat — are prepared, inspected and processed.
Believe it or not, the glue on the back of Israeli stamps is from a kosher source and rabbinically supervised.Glue made from horse or shellfish,for example, would be unkosher and,under the strcitest interpretation of Jewiush law, people would not be allowed to lick the glue side of a stamp. Very pious Jews outside Israel do not actually lick stamps before attaching them to envelopes.The same hold true for the envelope itself.Since,the glue,however,is not considered food, MOST normative orthodox Jews are not so strict.
There certainly are glues made from animal products and other non-kosher ingredients, such as those glues made from boiled animal connective tissue. However, glue that becomes sticky when moistened is generally made from different types of starches, generally derived from plants. If we were talking about food, we would certainly want it to be certified kosher. But, did anyone ever certify postage glue as kosher?
It seems that at one time, postage stamp glue in Israel was certified kosher. AFAIK, postage stamps are nearly all self-adhesive now.