SMTP

RFC 821 RFC-821 and RFC 1123, RFC-1123, specify this transfer protocol. This is the communications protocol used most commonly over TCP/IP links. SMTP usually operates directly between the source and destination machines, so intermediate machines don't get involved (except for gateways, see below). SMTP is usually implemented as part of the MTA. Extensions to SMTP are specified in RFCs 1425 RFC-1425, 1426 RFC-1426, and 1427 RFC-1427.