http://www.rfc-editor.org/rfc/rfc2487.txt
RFC 2487: SMTP Service Extension for Secure SMTP over TLS, P. Hoffman1999-01-01T23:00:00-00:00This document describes an extension to the SMTP service that allows an SMTP server and client to use transport-layer security to provide private, authenticated communication over the Internet. This gives SMTP agents the ability to protect some or all of their communications from eavesdroppers and attackers. [STANDARDS-TRACK]