When an envelope is stubbornly stuck and you must unstick it, several tricks may work to accomplish this task. Often, envelopes become stuck when high humidity levels activate the gum adhesive on the envelopes. You can unstick envelopes with simple household appliances. Once you successfully unstick the envelopes, they should adhere again normally when you are ready to use them.
Set the iron to low heat. When the iron is warm, run the iron over the sealed envelope to warm it slightly. Insert the tip of the knife between the two layers of the envelope to separate them. Slide the knife along the edge of the envelope flap to separate the flap of the envelope.
Set the iron to low heat. Moisten the clean cloth slightly with water. Wring it out well. Place the damp cloth over the envelope and place the warm iron over the cloth. Leave the iron on the cloth for two seconds and remove the iron. Insert the tip of the knife between the two layers of the envelope to separate them in the same fashion as in step one. This method may work better for more stubbornly-stuck envelopes because it uses moisture to unstick the envelope flap.
Place the stuck envelope into the microwave oven. Place a mug of water in a back corner of the microwave to add moisture. Turn the microwave on and run it for up to 20 seconds. Remove the envelope and open the flap. It should unstick easily.
Place the stuck envelope in the freezer. Leave the envelope there for up to 24 hours. The extreme cold and dryness of the freezer should unstick the envelope. Remove the envelope from the freezer and insert the tip of the knife between the two layers of the envelope to separate them. The envelope should unstick easily.