Method 1 - Freezing
Put all the envelopes on freezer for two hours. It freezes the adhesive and a slight flip on the envelope flap with a knife will help. You can put inside a plastic cover before putting it inside the freezer, if you worry about the moisture.
Method 2 - Steam
Iron the flap of the envelope with steam mode and pry to open the seal. This is the improved version of using water steam, but sometimes steam may make the envelope warped.
We can also use the steam from other sources like kettle or boiling water. I used steam iron because it is accurate on the flap and less effort. However this method is time consuming.
Method 3 - Microwaving
Put all your envelopes in a microwave for 10-15 secs and slight flip on the flap of the sealed envelope is enough. This is good method and I have tried. Do not crowd the microwave with envelopes, make sure adhesives are exposed to microwaves.
Microwave may burn any paper if it is thin and over heated, So just monitor it and increase +5 secs from 10 secs if needed. Use on own risk
Method 4 - Duct tape
If you do not want the envelope for any reference and not worried on the duct tape cost, then stick some section of duct tape on the envelope and peel it off fast. It will make an opening in the envelope and so you can tear the envelope apart fully.