steam it, open the seal gently, and reseal it? open it destructively and make a new envelope? this is probably the easiest option for a 3-letter agency. soak

steam it, open the seal gently, and reseal it? open it destructively and make a new envelope? this is probably the easiest option for a 3-letter agency. soak it in a liquid that doesn't dissolve the ink, photograph the layers of text, and decipher it later? shine a really bright light on it, using a color that isn't well absorbed by the paper but is absorbed by the ink? If there's a way to harden paper mail against even a moderately prepared adversary I'm totally interested in learning about it...