Ok, but if you're an Austro-Hungarian or German postmaster, and you have some number of letters going to French or British recipients, and somewhere there's a shifting front line of trenches and tanks and mustard gas, can you really roll up with mail bag to some quiet point and exchange it for incoming mail with some French or Italian counterpart? That seems like even if there wasn't a system of censors trying to make sure no military secrets are leaked, it would have required a great act of trust that no one on the other side had decided to mail a bomb or poison or at least instructions to a saboteur.