ISO/IEC 10021-6:2003 specifies the MTS Access Protocol (P3) used between a remote user-agent and the MTS to provide access to the MTS Abstract Service defined in ITU-T Rec. X.411 | ISO/IEC 10021-4. It also specifies the MS Access Protocol (P7) used between a remote user-agent and a message-store (MS) to provide access to the MS Abstract Service defined in ITU-T Rec. X.413 | ISO/IEC 10021-5. This document also specifies the MTS Transfer Protocol (P1) used between MTAs to provide the distributed operation of the MTS as defined in ITU-T Rec. X.411 | ISO/IEC 10021-4 and identifies the other Recommendations/International Standards which define other aspects of Message Handling Systems.