Simple Mail Transfer Protocol (SMTP - (IETF RFC 2821, Obsoletes RFC 821, RFC 974 and RFC 1869) most widely adopted protocol for sending and receiving e-mail, though more often used only for sending due to its limited ability to queue messages at the receiving end, J. Klensin, Editor, April 2001
x.400 - set of standards specified by the ITU-TS (International Telecommunications Union - Telecommunication Standard Sector), which an alternative to SMTP and common in Europe and Canada