What milestones shaped secure email (TLS, PGP, S/MIME)?

PGP (Pretty Good Privacy) appeared in 1991 and brought strong encryption to the public. Its creator Phil Zimmermann was investigated by the United States government because strong encryption was considered a weapon for export at the time. The case was eventually dropped but it became a landmark for privacy.

S MIME brought encryption and signing into corporate and enterprise environments. TLS (Transport Layer Security) made it possible to encrypt the connection itself so messages could not be read in transit.

It was like reinforcing every ship with armor and sealing every letter with a signature that could be verified at the next port.