Secure Multipurpose Internet Mail Extensions

Definition
Secure Multipurpose Internet Mail Extensions (S/MIME) is

"the recommended protocol for email authentication and confidentiality. S/MIME is particularly useful for authenticating mass email mailings originating from mailboxes that are not monitored, since the protocol uses PKI to authenticate digitally signed messages, avoiding the necessity of distributing the sender's public key certificate in advance. However, S/MIME senders need to possess the certificate of each recipient if the sender wishes to send encrypted mail. Research is underway that will allow the DNS to be used as a lightweight publication infrastructure for S/MIME certificates."