The present document specifies requirements for achieving interoperability between the Registered Electronic Mail systems that are compliant with TS 102 640 (REM henceforth) specification ETSI TS 102 640-1 to ETSI TS 102 640-5 and systems that are compliant with UPU S52-1 UPU Postal Registered electronic Mail functional specification (PReM henceforth). The approach used for this purpose is to define all the necessary mappings between the two specifications taking into account also the objective to maintain and preserve the positive features present in both the realities as pursued in the Technical Specifications.