How often does the military screen snail mail?

One of my friends and I were discussing whether or not we think the military screens snail mail that it’s soldiers send to friends and family for sensitive information. I thought they did, but my friend has recieved letters from other friends of his in boot camp and in the actual service and he says that the letters weren’t tampered with, and the handwriting on the addresses was theirs. So I’m curious. Does the military in fact screen letters? If so, is it only a selective screening based on what the enlistee happens to be doing at the time?