Are you a vegetarian? Are you the type of vegetarian that does not eat eggs? If you are, then be aware that the glue on the envelopes may contain materials made from eggs.
I never thought about this till today when my vegetarian colleague was trying to mail something and then he was looking for a glue stick to glue the envelope. I told him that you can just leak the glue on the envelope with your tongue and then press on it to stick, once he said, what if the glue contained eggs?
Well, if you are not that detail oriented and you are still a vegetarian, you might want to go backwards and check if you have broken your vegetarianism by leaking the glue on the envelope or some other similar activity.