It means that the issuer has a credible signal of such communication, which generally includes the evidence of its transmission, the content communicated, the dates and times of transmission, delivery and opening, as well as the identification of the parties through the addresses of origin and destination.
An exact copy of the content of the certified e-mail is guarded and can be used as evidence in a trial together with the notarial act of the deposit and an expert report or sworn statement (affidavit), thus providing all the data about its delivery, its receipt, the content sent, the date and the time.