1. S/MIME (Secure/Multipurpose Internet Mail Extensions)
   • Confidentiality: Achieved through encryption.
   • Integrity: Ensured by using PKCS (Public Key Cryptography Standards) and X.509 PKI.
   • Non-repudiation: Provided through signed message digests, ensuring the authenticity and integrity of the email.
2. PEM (Privacy Enhanced Mail)
   • Encryption: Uses AES (Advanced Encryption Standard) for securing email content.
   • PKI X.509: Utilizes X.509 certificates and RSA for public key infrastructure, providing secure email communication.
3. Message Security Protocol
   • Usage: Primarily used in military communication (X.400).
   • Security Features: Involves signing, encrypting, and hashing messages to ensure secure communication.
4. Pretty Good Privacy (PGP)
   • Encryption: Uses the IDEA (International Data Encryption Algorithm) for encrypting email content.
   • Public Key: Employs RSA for managing encryption keys and ensuring secure email communication.

These protocols and methods ensure that email communication is secure, maintaining confidentiality, integrity, and authenticity in various environments, from general use to military-grade security.