Secure Multipurpose Internet Mail Extensions (S/MIME) provides a secure method of sending e-mail and is incorporated into many popular e-mail applications. S/MIME provides confidentiality and authentication by using the RSA asymmetric key system, digital signatures, and X.509 digital certificates. S/MIME complies with the Public Key Cryptography Standard (PKCS) #7 format and has been proposed as a standard to the Internet Engineering Task Force (IETF).

In other words, the sender of an e-mail can provide a guarantee to the recipient that the e-mail is in fact sent from the sender, and that the content of the e-mail has not been tampered with on the way to the recipient. S/MIME relies on a system of public/private keys and trust authorities.