The present document specifies the interoperability profiles of the Registered Electronic Mail (REM) messages according to the formats defined in ETSI EN 319 532-3 [6] and the concepts and semantics defined in ETSI EN 319 532-1 [4] and ETSI EN 319 532-2 [5]. It deals with issues relating to authentication, authenticity and integrity of the information, with the purpose to address the achievement of interoperability across REM service providers, implemented according to the aforementioned specifications. The present document covers all the options to profile REM services for both styles of operation: S&N and S&F. The mandatory requirements defined in the aforementioned referenced REM services specifications are not normally repeated here, but, when necessary, the present document contains some references to them. More specifically, the present document: a) Defines generalities on profiling. b) Defines constraints for SMTP profile.