Method for the certification of electronic mail delivery from a transmitting user to a recipient, the method comprising carrying out in a certification system for electronic mail delivery which is implemented by a telecom operator, being the transmitting user a client of said certification system for electronic mail delivery, said certification system for electronic mail delivery comprising at least a data processing unit operative at least as an incoming mail server and an outgoing mail server which are interconnected, the following steps: - sending an electronic mail from the transmitting user to at least one destination electronic address of the recipient through an initial route by means of an initial mail server and a destination mail server, - sending, from the transmitting user, a copy of the electronic mail sent in the previous step to the certification system for electronic mail delivery through a second route, - receiving said copy of the electronic mail at the incoming mail server of the certification system, - inserting a particular indication in said copy of the electronic mail by means of the processing unit wherein the particular indication comprises the sentence CERTIFIED INVOICE, - sending through the outgoing mail server a second electronic mail which comprises the copy of the electronic mail with the particular indication, - delivering to the destination mail server the second electronic mail which comprises the copy of the electronic mail with the particular indication, - delivering to the recipient: - the electronic mail through the initial route, - the copy of the electronic mail through the second route, wherein said copy of the electronic mail comprises the particular indication, - receiving in the incoming mail server data relative to the delivery of the second electronic mail from the outgoing mail server, - generating at the processing unit an electronic document comprising data related to the preceding steps, - applying a digital signature to the electronic document of the previous step for the creation of an electronic certificate, - sending from the outgoing mail server the electronic document to a third party to carry out a second digital signature, and - delivering the electronic certificate to the transmitting user from the processing unit. - sending an electronic mail from the transmitting user to at least one destination electronic address of the recipient through an initial route by means of an initial mail server and a destination mail server, - sending, from the transmitting user, a copy of the electronic mail sent in the previous step to the certification system for electronic mail delivery through a second route, - receiving said copy of the electronic mail at the incoming mail server of the certification system, - inserting a particular indication in said copy of the electronic mail by means of the processing unit wherein the particular indication comprises the sentence CERTIFIED INVOICE, - sending through the outgoing mail server a second electronic mail which comprises the copy of the electronic mail with the particular indication, - delivering to the destination mail server the second electronic mail which comprises the copy of the electronic mail with the particular indication, - delivering to the recipient: - the electronic mail through the initial route, - the copy of the electronic mail through the second route, wherein said copy of the electronic mail comprises the particular indication, - the electronic mail through the initial route, - the copy of the electronic mail through the second route, - receiving in the incoming mail server data relative to the delivery of the second electronic mail from the outgoing mail server, - generating at the processing unit an electronic document comprising data related to the preceding steps, - applying a digital signature to the electronic document of the previous step for the creation of an electronic certificate, - sending from the outgoing mail server the electronic document to a third party to carry out a second digital signature, and - delivering the electronic certificate to the transmitting user from the processing unit.