http://www.ietf.org/rfc/rfc1204.txt
Operating systems for personal computers do not provide a mechanism for user authentication. However, such a mechanism is crucial for electronic mail system since authenticating message sender's identity is important in preventing mail forgery. Hence, adding personal computers to an electronic mail network requires an agent (message posting server) to authenticate sender's identity and then submit mail to the message delivery system (e.g., Sendmail, MMDF) on behalf of the sender at a PC. The Netix Message Posting Protocol is developed to be the interface between the message posting server and the PC (client). The protocol is designed to use TCP and is based on the command and reply structures defined for Simple Mail Transfer Protocol (RFC 821) and File Transfer Protocol (RFC 959).