[varnaud]

Snail mail is still scanned for illicit content. Also many times in western history letters were read and then possibly censored at processing facilities. Back in those days, it was a human opening letter/package and inspecting the content. They had rules to respect the privacy of the sender/addressee, like not sharing the content with anyone else if the content is OK.

I'm not sure who considered these services to be one of the most private.

Also in those days, one could make use of a secure private mail service, bypassing the normal processing facilities and have grantee that the letter remained sealer the whole journey. To be even more secure, basic encryption could be used, or even invisible ink.

Today, things just got more sophisticated.