This Technical Report recommends a set of Guidelines to ensure interoperability at the transmission level to be used in conjunction with the European Norm (EN) for the semantic data model of the core elements of an electronic invoice and its other associated deliverables. The Guidelines are by nature non-prescriptive and non-binding. These Guidelines take into account the following aspects: 1) recommending best practices for use at the transmission level; 2) supporting interoperability between all the parties and systems that need to interact and within the various operating models in common use; 3) ensuring a level playing field for the various operating models and bi-lateral implementations and for the use of existing and future infrastructures, which support e-Invoicing; 4) promoting a common terminology and non-proprietary standards for transmission and related areas; 5) ensuring the authenticity of origin and integrity of electronic invoice content; 6) providing guidance on data protection, on the enablement of format conversion, and on e-invoice legibility, including the use of a readable visual presentations, as required; 7) providing guidance for identification, addressing and routing; 8) identifying requirements for robust legal frameworks and governance arrangements; 9) recognizing the roles of trading parties, solution and service providers and related infrastructure providers. The Objectives of the Guidelines are: 10) to support the implementation of the EU Directive 2014/55/EU on e-Invoicing and the core invoice model; 11) to propose best practices and recommendations for standards to enable electronic exchange of e-Invoices and related data between participants by providing a basis for interoperability at the transmission level, based on common requirements and scenarios; 12) to facilitate Straight Through Processing (STP) by the key actors in the supply chain (Buyers, Sellers, Tax Authorities, Agents, Banks, Service and Solution Providers, etc.); 13) to provide a set of non-prescriptive and non-binding Guidelines and recommendations that are applicable to all common operating models for e-invoice exchange and transmission whilst also providing recommendations specific to each of the common operating models. To accomplish these objectives, the Guidelines are based on the following Requirements and Guiding Principles: 14) the need to cover the transmission of e-invoices and related documents from the system of the sending trading party to the system of the receiving trading party, including transmission issues for any intermediary platforms; 15) the need to allow any seller in any European (EU, EEA and Switzerland) country to deliver invoices to any buyer in any location in another European country (EU, EEA and Switzerland); 16) the need to support all common invoicing processes and modes of operation; 17) the need to be compatible with the current legislative and regulatory environment for the exchange of e-Invoices and related data; 18) the need to support the European Norm and other commonly accepted content standards; 19) the need to ensure that other document exchanges beyond e-Invoicing can be supported; 20) the need to establish clear boundaries between the collaborative and competitive domains; 21) the need to enable competition between business models, solutions and service providers and foster innovation; 22) the need to ensure that European supply chains remain an integrated and competitive part of the global economy; 23) the need to promote network effects leading to the development of critical mass as e-Invoicing becomes the dominant mode of invoicing (network effects result in a service becoming more valuable as more trading parties use it, thus creating a virtuous circle and further momentum for adoption). The following items are considered to be in the competitive domain and therefore out of scope of the Recommendations: (....)