S/MIME stands for Secure/Multipurpose Internet Mail Extensions and is a standard for encrypting and signing email messages. It uses public key cryptography to ensure the confidentiality, integrity, and authenticity of email communications. FTPS is a protocol for transferring files securely over SSL/TLS, but it is not used for emailing. DNSSEC is a protocol for securing DNS records, but it does not protect email content. AS2 is a protocol for exchanging business documents over HTTP/S, but it is not used for emailing.