All electronic invoices must be sent and received via Sistema di Interscambio (SdI). The system receives all invoices and, after a quick check, it delivers them to clients. That's the reason why a client has to type in the electronic invoice the right address where to recive electronic invoices.
You can receive your electronic invoice via PEC, if in the invoice is specified the "PEC Receiver" field or via a a reception system using a 7 characters code, released on a SdI request, which has to be typed in the "Codice Destinatario" field. This last method is definitely easier to manage for the user, who doesn't have to monitor the PEC delivery notifications constantly.
To ease the delivery process of electronic invoices, Agenzia delle Entrate provides you with a pre-registration service where you can previously upload your e-mail address, PEC or Codice Destinatario (Codice intermediario provided by Keliweb), where to receive all electronic invoices automatically.
To use this service, enter the "Electronic Invoicing" section of the "Fatture e corrispettivi" portal and then continue on "Register your e-mail address to receive your electronic invoices". You will be able to delete or change your address at any time.