http://www.rfc-editor.org/rfc/rfc0821.txt
RFC 0821: Simple Mail Transfer Protocol, J. Postel1982-08-01T23:00:00-00:00The objective of Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently. SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. Obsoletes RFC 788, 780, and 772.