Licking Stamps17 Kislev 5772
Is there a halachic issue licking the glue of an envelope?
I don’t know of such a halachic issue, and the substance is certainly pagum, so that there is no halachic concern in licking.
Nonetheless, some, due to not knowing what the glue might contain, prefer to use a finger rather than directly licking the envelope/stamp. However, there is no halachic need to do this.