“Electronic system for exchanging digital registered mail” Description Field of the invention The present invention relates to the field of digitization of mail addresses. In particular, it relates to the sending of registered mail via digital means and with possibly multimedia contents. Prior art Many methods for fully electronic delivery of registered mail have been published previously. However, a simple return receipt service does not always perform the same functions as the equivalent mail service. The missing piece consists of the legal guarantees that accompany the latter and that allow the use thereof by the public administration, for example. An important detail therefore, which escapes the solutions proposed so far, is that of an integration with other public services of certified digital data, which, although different in the various countries, we will try to outline in the following description, thus proposing in the present invention a framework for the implementation of a digital registered mail service. Patent CN102202006A describes a method of sending registered mail with a read receipt which, however, is implemented only through electronic messaging and therefore does not have the security guarantees provided with the present invention such as the fact that the reading by the recipient is authenticated by public identity systems and is made with a physical print of the message that cannot be subjected, once produced, to cyber-attacks. Similarly, the Korean patent KR20110071782A uses cryptographic keys to guarantee the security of the exchange of a registered letter but does not specify any method for a hybrid paper delivery legally easier to use in many contexts and countries. Finally, patent US2002130172A1 describes an electronically controlled mailbox which allows the exchange of registered mail. However, this patent does not address the legal problems that generally arise with these systems as they are instead addressed by the present invention. There is no doubt, therefore, that the aforementioned solutions only partially address the legal criticalities linked to the exchange of registered letters described up to now. Description of the invention In recent decades, the digitization of communications between people, companies and public administrations, made possible by ICT technologies, primarily the Internet, has resulted in a progressive shift of many communications on digital media, to the detriment of traditional paper communications. This progressive dematerialization (e-substitution) especially in the mail sector is becoming increasingly popular due to the obvious advantage it presents: the possibility, that is, of sending mail directly from home or from the office, avoiding the endless queues at post offices, and receive it guaranteeing the security and legal validity of the message exchanged. The present invention has focused interest on sending and receiving a registered letter via digital means. The registered letter is the sending of a communication with the certainty that it will not be lost and can be traced at any time, if one then adds the receipt service, one will receive a second communication from the post office which communicates the occurred delivery. Registered mail is sent from post offices and when it arrives at its destination, it must be delivered to the recipient or to the person in charge of receipt. If the recipient is absent, a held mail notification must be left at the recipient's address indicating the time, location and method of collection. Registered letters are used for all official communications, such as the notification of contracts between private individuals, public administration orders, delivery of judicial documents and any communication required by law between the legal parties. In fact, the burden of proof of sending and receiving rests on the person who has the legal obligation, the sender, and the signed delivery and return receipt constitute proof with legal value of this fulfillment. A digital registered letter is formally an electronic document which contains the computerized representation of legally relevant deeds, facts or data, meaning by electronic document “any content stored in electronic form”, whether it is text, sound, visual or audiovisual recording. The CAD (Digital Administration Code) attributes to this electronic document the probative effectiveness required by law (Article 2702 of the Italian Civil Code), or the effectiveness of private writing, when there is a digital signature, or other type of qualified or advanced electronic signature, or when it is possible to identify the author of the document and the same has been created through a certain process that is able to guarantee the security, integrity and non-modifiability of the document and its unequivocal traceability to the author. In detail, the CAD governs digital identity, digital domicile and access to digital services, allowing simplifications in order to facilitate access to public administration network services by citizens and businesses and the effective exercise of the right to use digital technologies. The Decree attributes central value to the digital identity SPID and CIE (electronic identity card) and to the PEC as an implementation tool of the so-called “Digital Domicile”, the right of which had already been recognized by art.3 bis of the DAC, but, up to now - in fact - implemented only for those categories (professionals and companies) already obliged to have a PEC entered in the public registers. It is therefore evident that in order to implement a complete digitalization service of the registered letter that does not only concern digital collection, but also digital sending, this involves the need to prepare a certified and qualified electronic delivery service system. The present patent provides a communication service by registered letter between a sender and a recipient using digital technologies which replace the traditional paper registered letter, but which, at the same time, guarantee the legal validity of the communication by interfacing with public digital documentation systems. To create the digital registered letter, this system therefore makes use of a digital signature device, which, as we have seen, constitutes the basic element for the legal validity of the electronic document, which allows the digital signature to be affixed to the message sent via recommended to guarantee the authenticity and non- repudiability of the message. The recognition of the sender legal entity is guaranteed by a provider adhering to a public digital identity system with which the present system interfaces. Another fundamental step of this digital communication is the digital domicile of both the recipient and the sender. To this end, the system has a second software interface with a digital domicile provider which ensures that the digital and physical addresses of the sender and of the receiver of the message are registered and legally connected to the respective legal subjects. An internet connection guarantees communication with the aforementioned interfaces to the present system. The sender, through the present digital sending system of the registered letter, has a panel available where he/she can proceed with the digital sending: which consists in uploading the document, indicating the digital address and monitoring the progress of his/her shipment through the portal. If the authenticity of the sender is guaranteed by the digital signature, recognized by the public digital identity system, to guarantee the legal validity of the delivery of the digital registered letter to the recipient, the present system provides for a printing device with location and authentication of the recipient through a GPS geolocation system, a biometric identity reader and a printer. The processor of such printer connected via internet connection to the GPS system carries out the activity of printing the content of an electronic message sent only if the GPS detects the position corresponding to the address of the recipient, but not only, to allow printing and therefore the actual delivery, there is an additional security check which is biometric recognition. In fact, the system at the recipient’s is provided with a biometric reader, which can make use of retinal or fingerprint recognition, such recognition gives the printer the authorization to proceed. Therefore, if the printing process is successful, the digital registered letter can be considered legally delivered. The text of the printed electronic message may contain only part of the content of the digital registered letter, the abstract or the subject in general, while the multimedia files associated with it can be opened and consulted digitally. These files can be comfortably consulted by virtue of the presence of a touch display which allows the user not only to open and read the digital registered letter, but also to receive notifications and interact with the sender. Since the registered letter is an official document in all respects, the need to guarantee the authenticity of the credentials is the basis of the effectiveness of such system, for this reason it is provided with both an electronic identity card reader and a scanner for uploading paper identity documents, necessary for the legal validity process of the registered letter. The electronic system for exchanging digital registered mails provides for an exchange of data between the sender, the recipient and the set of public services with which one must interact to ensure an effective exchange of communications, such data exchange takes place in a decentralized manner, through Blockchain technology, in order to avoid the drawbacks of a central server vulnerable to cyber-attacks and disruptions. To obviate also technical impediments such as the interruption of electricity, the system is provided with an emergency power supply unit which guarantees the operation of the system and the reception of important messages even in situations of absence of electricity. The peculiar feature of a registered letter is traceability and its unique identification, in this regard this system produces an identification QR code containing the output of a hash function applied to the message sent digitally. This two-dimensional code applied and sent by conventional registered mail produces an arrival notification of the actual message contained in the digital delivery system. The advantages offered by the present invention are clear in the light of the above description and will be even more apparent from the accompanying figure and the related detailed description. Description of the figures The invention will hereinafter be described in a preferred embodiment thereof by way of non- limiting example with the aid of the accompanying figure, in which: - FIGURE 1 shows the main constituent elements of said digital registered letter exchange system which, using electronic equipment, enables the registered letter service between a sender and a recipient and guarantees the legal validity of the communication by interfacing with public digital documentation systems. The invention comprises several components: a device for the digital signature 100 adapted to affix said digital signature to the message and two software interfaces, one characterized by a provider adherent to a public digital identity system 102 which ensures that the sender legal entity is identified, and the other, on the other hand, comprises a digital domicile provider 104 adapted to ensure that the digital address of the receiver of the message is registered and legally connected to the recipient. In order to guarantee and solve the problem of the legal validity of the delivery, said system consists of a printing device 105 composed of four different modules: a GPS geolocation system 106, a biometric identity reader 107, a printer 108 provided with a processor 110 and an Internet connection 109 for communication through said software interfaces. Said device is also integrated with a touch display adapted to allow interaction with the user 300 and a scanner 400 connected to the printing device 105 for loading the paper identity document. The system also provides for the production of a QR code 700 sent by conventional registered mail to indicate a notification of the arrival of the message. Finally, the figure illustrates some integrative elements of said system such as: an electronic identity card reader 200 suitable for constituting an alternative authentication system with respect to said biometric reader 107 and an emergency power supply unit 600 adapted to ensure the operation and the reception of messages in all kinds of situations. Detailed description of the invention The present invention will now be described purely by way of non-limiting or binding example with the aid of the figure, which illustrates an embodiment relative to the present inventive concept. With reference to FIG. 1, the main components constituting the present invention are illustrated. Said digital registered letter exchange system comprises a plurality of components: a digital signature device 100 adapted to affix said digital signature to the message sent by registered letter to guarantee the authenticity and non-repudiability of the message and a dual software interface, the first is associated with a provider adherent to a public digital identity system 102 which ensures that the sender legal entity is identified, while the second with a digital domicile provider 104 adapted to ensure that the digital address of the recipient of the message is registered and legally connected to the recipient, and the same is possible for the sender whose digital address is required for the acknowledgment of receipt. Said system, in order to solve the problem of the legal validity of the delivery, proposes a printing device 105 with localization and authentication of the user comprising in turn: a GPS geolocation system 106, adapted to ensure that the printing of the message takes place in the coordinates corresponding to the physical domicile of the recipient obtained through said interface with a digital domicile provider 104, a biometric identity reader 107 adapted to allow authentication of the recipient of the message or of the acknowledgment of receipt using the data obtained from said public digital identity system 102, a printer 108 whose processor 110 performs the functions of printing and communicating via the Internet with said services and, finally, an Internet connection 109 adapted to allow communication via said software interfaces. Said printing device is also characterized by the presence of a touch screen display adapted to allow interaction with the user 300 and a scanner 400 connected to said printing device 105 for the process of loading the paper identity document. The operation of said device consists in printing a copy of the content of the electronic message in the event that the GPS detects a position corresponding to the recipient's home and subject to the authorization of the recipient through biometric recognition. Said registered letter can therefore be considered legally delivered only if the printing process is successful. Said digital registered letter exchange system also provides for the production of a QR code 700 sent by conventional registered letter to indicate an arrival notification of the actual message contained in the digital delivery system, and containing the output of a hash function applied to the message sent digitally, thus ensuring that the message has not been forged and that the sender has actually sent it according to a legally valid process. Finally, some supplementary modules of said system are proposed, which provide a reader for electronic identity card 200, adapted to represent an alternative authentication system and an emergency power supply unit 600 adapted to ensure the operation and reception of important messages also in situations of absence of electricity. Finally, it is clear that modifications, additions or variants may be made to the invention described thus far which are apparent to those skilled in the art, without departing from the scope of protection that is provided by the appended claims.