“Secure / Multipurpose Internet Mail Extensions”
S / MIME is a certificate-based encryption solution that allows you to both encrypt and digitally sign a message. Message encryption helps you ensure that only the intended recipient can open and read the message. A digital signature helps the recipient to verify the identity of the sender.
Both digital signatures and message encryption are made possible through the use of unique digital certificates that contain the keys for verifying digital signatures and encrypting or decrypting messages.
To use S / MIME, you must have public keys for each recipient. Recipients must manage their own private keys, which must be permanently secured. If a recipient’s private key is compromised, the recipient must obtain a new private key and distribute new public keys to all potential senders.

Image source: https://www.psw-group.de/blog/sicherheit-kommuniegen-psw-group-schuetzt-ihren-e-mail-verkehr/3031