you should read about standard intelligence operations and how underground resistance organizations have historically operated ie. French resistance in Nazi-Occ

you should read about standard intelligence operations and how underground resistance organizations have historically operated ie. French resistance in Nazi-Occupied France, etc. for a case study in covert comms. the key you pointed to tho was regarding convenience. simple fact is that most of the usability desired in consumer email is not compatible with the practical design principles of covert communications.