RFC 5751: Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
Defines S/MIME 3.2 for signing and encrypting email using X.509 certificates.
Encryption
RFC 8446: TLS Protocol Version 1.3
The current TLS standard. Mandatory reading for secure SMTP (STARTTLS/SMTPS) and IMAP/POP3 over TLS.
