Just place your envelope inside a folded towel, and run the iron over it in a precise quick motion. Do some testing to your progress and focus on the parts of the envelope that haven’t been opened yet and check your progress.
After some work, the envelope should pop open. This method works great because you can actually re-seal the envelope using the glue that was already there. The only thing you need to do is place the letter back into the envelope and then run the iron over it in the paper towel without the steam this time.