Cryptography - Encryption (Enciphering) in Email
Message can be encrypted using:
S/MIME is a long-standing protocol which allows encrypted and signed messages to be sent using standard email delivery SMTP.
It uses public key cryptography to:
S/MIME 1) employs a model based on a trusted certificate authority (CA) that signs users' public keys.
Note: A message can't be decrypted if the user's key isn't uploaded when the message is delivered. Learn more about uploading certificates.
PGP (Pretty Good Privacy) is a protocol that uses also the public key cryptography to encrypt email.
See https://emailselfdefense.fsf.org/
SSL (TLS) is a protocol that helps provide privacy between communicating applications and their users during email delivery. When a server and client communicate, TLS ensures that no third party can overhear or tamper with any messages.
For delivery TLS to work, the email delivery services of both the sender and the receiver must always use TLS.
See Postfix - TLS (SSL) configuration 3)