Abstract A large number of certified mail systems have been put into operation on the market over the last years. In contrast to standard mailing systems like e-Mail, certified mail systems provide the secure, reliable and evidential exchange of messages with the quality of traditional postal registered or certified mail. Most of these systems are tailored to national laws, policies, needs and technical requirements and are thus closed and only accessible by certain user groups. However, the ongoing globalization and opening of the markets, especially in the European Union, ask for global certified mailing as already known from e-Mail. Interoperability of certified mail systems is a new and challenging research field. This article presents a framework and standard to make arbitrary certified mail systems interoperable. The presented approach uses a federated trust network of so-called electronic delivery gateways for seamless certified mailing across systems. This is achieved by converting protocols and system specifics on different layers using a harmonized interoperability protocol. The presented framework has been standardized by the European Telecommunications Standards Institute (ETSI) as Registered Electronic Mail specification for interoperable certified mail systems.