This Technical Report consists of three parts.The first part defines, describes and explains basic concepts typical to all mail communication systems such as; domains, parties, agents and their role in the system, physical and informational objects, processes, interfaces and relationships. The first part does not cover more detailed technical aspects of the main concepts such as:- a detailed description of mail units and sets, their attributes and methods of collection/capture of their values; - applications describing specific use of the information describing basic objects by mailers, postal operators and recipients;- data elements, data constructs and me