Specifies the message transfer system (MTS) access protocol (P3) used between a remote user-agent and the MTS to provide access to the MTS abstract service and specifies the message store (MS) access protocol (P7) used between a remote user-agent and an MS to provide access to the MS abstract service; it also specifies the MTS transfer protocol (P1) used between message transfer agents to provide the distributed operation of the MTS. This Standard is identical with and reproduced from ISO/IEC 10021-6:1990, and includes Technical Corrigendum 1:1991, Technical Corrigendum 2:1991 and Technical Corrigendum 3:1992.