Back in late medieval europa most postal services had a back room, called
cabinet noir where letters where carefully opened, read and resealed to check them for signs of treason against the crown or cross. These were often abused for what we today would call economic espionage. For these reasons many rich people employed private couriers who traveled to their business partners in person to hand over messages.
Those who could not afford such luxury had to rely on codes. The prevalence of such codes however increased the chance of being misunderstood by those who searched for treason. When the emerging middle class started to demand political power in the renaissance it was answered with violence and repression. Those who wrote the constitutions of all modern western nations understood that democracy can only exist when the state has no business reading the correspondence of its citizens.
Today many argue the state has a need to access such correspondence to prevent crime, but such a need is like the need of an addict: nothing good can come from it and the people should not enable these institutions to satisfy an ever growing demand for insight into their private lives. One must remember that democracy is founded on the believe that thoughts and words are not crimes and everyone must be free to express them-self in public, but even more so in private correspondence. A society that mistrusts its own citizens to a point where all those that whisper to each other are called criminals, dealers, traitors or terrorists is rotten at its core.
And yet some still say: but if the state can read all private correspondence it would be so much easier to catch criminals. And yes, it is true that these totalitarian methods ar efficient in fighting street level crime. However for society as a whole, such methods enable a terror of the state that is a crime against humanity itself. They say "but the state will never abuse its power" and i say: it did countless times before. Do not stray away from liberty and freedom for promises of safety made by those that profit from oppression.