http://www.rfc-editor.org/rfc/rfc2033.txt
RFC 2033: Local Mail Transfer Protocol, J. Myers1996-10-01T23:00:00-00:00SMTP [SMTP] [HOST-REQ] and its service extensions [ESMTP] provide a mechanism for transferring mail reliably and efficiently. The design of the SMTP protocol effectively requires the server to manage a mail delivery queue. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.