Matt Novak has published an interesting article at https://goo.gl/VzJc9d describing how the CIA always sends old-fashioned snail mail through the postal system. It seems the CIA sends out letters that are secured with a specific type of tamper-proof tape.
The “gummed Kraft sealing tape” the agency uses makes sure the letter or package you receive has not been surreptitiously opened along the way. There’s no way to open the envelope without making it clear someone has been messing with it. The tape cannot be steamed off and has laminated glass fibers embedded in the tape to resist cutting.
Using this tape on your letters will help protect your privacy by making sure no one messes with your letter or steams open the envelope to steal the check you sent inside.