Interoperability can be defined as a capability that facilitates reliable communication between products from different manufacturers without requiring conversion of the data by proprietary interfaces. The creator of a signed or encrypted e-mail therefore does not need to know about the components used by a communication partner in order to create data formats the latter can utilise. The interfaces (protocols and formats) are defined by internet standards (RFCs (Requests for Comments) that are made available to software developers. In its decision of 16 January 2002, the German Federal Government defined this context as 'horizontal interoperability'.

Communication partners experience interoperability problems despite an extensive set of standards. Messages cannot be properly displayed and verification processes fail.

Quality assurance tests for interoperability were conducted as part of the Sphinx project. This testing made sure that the PKI (Public Key Infrastructure) products and trust centres deployed during field trials conducted across Europe were interoperable with one another. By establishing a PKI for public administration use, plus the accompanying large-scale rollout of PKI products for secure e-mail communication to administrations at federal and state level, a further set of functionalities and associated interoperability problems then needed to be addressed. The interoperability tests were then continued from December 2001 to ensure that the products deployed were always interoperable.

As a result of interoperability testing, the Federal Ministry of the Interior, Building and Community recommends only procuring successfully tested products for use in the federal administration. The tests are performed quarterly on behalf of the BSI by a test lab operated by Atos Origin GmbH (formerly: SchlumbergerSema Competence Center Informatik GmbH) in Meppen, Germany. The tests are based around a test specification